devkuma – Network

Network Overview

kc@example.com (kc kim) — Sat, 26 Nov 2022 08:24:00 +0900

What Is a Network?

A network generally means “connecting people or things like a net.” In the world of IT systems, it means “interconnecting computers.” It may also be described as “a path through which information flows.” By connecting computers through communication circuits, cables, and similar media, it becomes possible to share information, distribute processing, and exchange messages.

The word network is a compound of Net + Work. If translated literally, it means “net work,” and in a more understandable way, it can be described as “work performed cooperatively while being connected to one another like a net.” Going further, it can be described as “work in which computers share resources while connected to one another.”

Then what is the Internet? It is a huge integrated communication network that connects networks around the world through a set of communication rules, the TCP/IP protocol.

Components of a Network

A network consists of “computers,” “network devices,” “transport media,” and similar elements. Computers are PCs, servers, and other machines that provide or use applications (services). Network devices, such as switches and routers, transport application data. Transport media, such as cables and wireless radio waves, connect computers and network devices.

Network Topology

Network topology refers to the physical connection form of various networks. It describes the overall shape of computers connected by cables in a network.

Physical structure: the actual connection shape of hosts connected by media.
Logical structure: the network shape based on how hosts access the media.

There are six forms of physical topology.

Bus Topology

Bus topology is a form in which each node is connected to a common line called a bus. All devices are connected to a central cable called a backbone. The Ethernet protocol also uses this form.

Advantages
- Nodes can be added and removed easily, and a failure in a specific node does not affect other nodes.
- It is inexpensive.
Disadvantages
- Because the bandwidth of the common line is shared, as the number of nodes increases, traffic on the line increases and network performance decreases.
- To expand it, the entire network must be stopped, and if a cable problem occurs, it is difficult to find.

Star Topology

As it looks, this topology is shaped like a star. Each node is connected to the center within 100 meters. Each node has input and output through only one link.

Advantages
- Failures are easy to find, and management is easy.
- Even if a node has a problem, the entire network is not affected.
- Expansion is very easy because you only need to connect to the center.
Disadvantages
- If the main node fails, the entire network becomes unavailable.
- Although management is easy, all transmission must pass through the center, so bottlenecks may occur.

Ring Topology

Ring topology is a circular topology in which each node is connected to its neighboring nodes on the left and right. Each node is directly connected to the device next to it. Data can be transmitted in only one direction. Token Ring also uses this form.

Advantages
- Because communication is one-way, signal amplification is possible and there are fewer distance limitations.
- It is easy to install over a wide area and has the advantage of being able to use wide bandwidth.
Disadvantages
- Adding and removing nodes is not easy.
- Installation cost is high. (It decreases if the space is small.)

Mesh Topology

Mesh topology is a form in which everything is connected to everything else. It is a net-like form where all nodes are connected one-to-one, allowing multiple pairs of nodes to communicate at the same time.

There are fully connected and partially connected forms. A fully connected network does not require switching or broadcasting, but the number of connections increases rapidly each time a node is added.

The number of connections required for n nodes is n(n-1)/2.

Advantages
- A failure in a specific node does not affect other nodes, and line failures can be handled flexibly.
- It also provides the strongest security.
Disadvantages
- Line construction costs are high, and adding a new node creates an additional cost burden.
- Installation and readjustment are difficult.

Communication Methods

Broadcast, unicast, and multicast are all communication methods.

Broadcast: sends a transmission message to an entire network.
Unicast: a one-to-one communication method.
Multicast: sends data only to selected recipients.

ISP (Internet Service Provider)

An Internet service provider.
For example, providers such as KT, U+, and SK Broadband.

Internet Overview

kc@example.com (kc kim) — Sun, 03 Apr 2022 17:26:00 +0900

What Is the Internet?

The Internet is a compound of Interconnect Network and means interconnected networks. It refers to a collection of networks in which countless computers around the world are connected. Computers connected to the Internet are linked using a communication protocol called TCP/IP (Transmission Control Protocol/Internet Protocol). Each computer that makes up the Internet has a unique computer address called an IP address, and they are connected to one another through communication methods such as cables or satellites.

Because of this, computer users around the world can easily obtain the information they need anytime and anywhere through the Internet.

History of the Internet

The beginning of the Internet goes back to 1969, when ARPANet (Advanced Research Project Agency Network) from the U.S. Department of Defense first appeared. Early ARPANet began as a network that connected the computer systems of four U.S. universities (Stanford, UCLA, UC Santa Barbara, and Utah State University), designed so that even if one system was destroyed during a nuclear war, the remaining systems could still connect and compensate perfectly. After that, it gradually expanded by connecting major U.S. government and educational institutions. In 1983, ARPANet was divided into MILNet, the part used for military purposes, and the Internet, the part used for civilian purposes.

Internet Addressing System

To exchange information on the Internet, each computer needs to be distinguished by assigning an address or name. The addressing systems provided by the Internet include IP addresses and domain names.

To maintain the Internet, Internet addresses must be managed, and an organization responsible for new Internet technologies and standardization is needed. NIC (Network Information Center) is an international organization that registers and manages network and computer addresses such as IP addresses and domain names. In Korea, KRNIC, an affiliated organization of NIC, is responsible for Internet address management.

IP Address

An IP address is a method of representing a computer’s address with numbers. It is structured by listing four areas in order, separated by dots (.), from the highest-level address. For example, Google’s IP address is represented as 172.217.174.100. However, this method is inconvenient to memorize or use, so domain names are commonly used.

Understanding URLs

All web documents on the Internet have a URL (Uniform Resource Locator), which is their own address and unique storage method. Just as each person has a unique fingerprint, every homepage has a unique URL. In other words, to retrieve data on the Internet, there must be a way to indicate where certain data is located. Therefore, a URL is a method for representing the location of information on the Internet. It has been used since 1990 and has now become the standard.

URLs are written using characters to identify items and specify addresses within a computer network. In other words, a URL provides location information for a document, and an Internet web browser opens the URL at that location and displays the document. Because most location information is written in highlighted links, users do not need to remember the URLs of each document or homepage.

LAN (Local Area Network) and WAN (Wide Area Network)

kc@example.com (kc kim) — Fri, 18 Nov 2022 11:01:00 +0900

LAN (Local Area Network)

LAN stands for Local Area Network.
It refers to building a network by connecting computers with dedicated cables within a relatively small area, such as an office or building.
This type of network can also be built at home, where it is called a home LAN or home network.

Types of LAN

By connection topology, LANs include star, bus, and ring types.
By communication control method, examples include Ethernet, FDDI, and Token Ring.
Today, star LANs based on Ethernet are the most commonly used.

WAN (Wide Area Network)

A WAN (Wide Area Network) is a network over a broad area. A network that connects LANs within an organization or across buildings is called a WAN.
It is managed by an Internet service provider.

Intranet

An intranet can be thought of as an internal company network. It is roughly a network made up of multiple LANs.
Only members inside the intranet can communicate and share information with each other.

Extranet

An extranet is a network that extends an intranet, where only internal members can communicate, to allow communication with approved external users.

Telnet

kc@example.com (kc kim) — Sun, 03 Apr 2022 18:02:00 +0900

Telnet

The Telnet service on the Internet is a service that lets you connect to another computer, especially one using the UNIX operating system, and use it as if a keyboard and monitor were connected directly to that computer.

It is an Internet standard protocol supported when connecting to a remote host computer through the Internet.
It is a TCP/IP-based protocol.
It is a remote terminal connection service.
TCP port number 23 is used by default.

It can display only a text screen rather than a graphical screen, but it is still widely used because it is simple, fast, and allows direct use of UNIX. However, to use Telnet well, you need to learn UNIX commands, and because only a text screen can be used, it is difficult to use without special knowledge.

Telnet Characteristics

Telnet is a kind of terminal emulation protocol and uses the concept of a virtual terminal called NVT (Network Virtual Terminal).
It has a one-to-one symmetric relationship between the terminal and the host.
Additional option negotiation is possible.

What Is NVT (Network Virtual Terminal)?

NVT is the most basic part of the Telnet protocol. When the remote system to connect to and the local system have different versions and therefore different data formats, it is a virtual device that converts data for smooth communication.
NVT defines basic terminal control characters.

How Telnet Works

The client establishes a TCP connection to the remote system (server) through remote login. (Default port 23)
The remote system provides a virtual terminal to the connected client.
The client executes commands on the remote system as if it were an actual terminal.
The remote system executes the client’s command and sends the result back to the client.

Telnet Connection Clients

Server and client built into Windows by default
- For security reasons, the feature is disabled by default. To use it, it must be enabled in settings.
Putty, SecureCRT, Xshell, and similar tools can be used conveniently.

References

Telnet protocol analysis and packet analysis (1)

Web

kc@example.com (kc kim) — Sun, 03 Apr 2022 18:02:00 +0900

What Is the Web?

The term most often encountered when using the Internet is probably the World Wide Web (WWW). The web is also called WWW, using only the first letters of the English words, or simply Web. The web symbolizes a web, meaning a spider web.

Because the web is implemented graphically, one of its greatest advantages is convenient information search. It can display text and graphics on the screen at the same time, and thanks to its excellent user interface, text, graphics, sound, video, and similar content can be handled easily in a browser.

Web Browser

A web browser is a client program that allows users to view hypertext documents on a web server using the Internet. It can process not only text but also multimedia data through hypermedia functions. When users want audio or video information, it can load and execute the required programs to handle various resources on the web.

Basic functions provided by a web browser include opening web pages, providing a list of recently visited URLs (Uniform Resource Locators), remembering and managing frequently visited URLs, and saving and printing web pages. Without a web browser, even if you are connected to the Internet, you cannot view homepages on the Internet.

Types of Web Browsers

Google Chrome.
Mozilla Firefox.
Chromium Microsoft Edge.
Opera browser
Vivaldi browser
Naver Whale
Brave browser

How the Web Works

Image source: http://tcpschool.com/webbasic/works

The user enters a URL in the browser.
The browser finds the server’s IP address through DNS.
It creates an HTTP request message using the HTTP protocol.
It sends the HTTP request message through a TCP/IP connection.
The server creates an HTTP response message using the HTTP protocol.
It sends the response to the requesting computer through a TCP/IP connection.
The arrived HTTP response message is returned as web page data and rendered by the web browser so the user can view it.

International Standards for Web Accessibility

Web standards designed to improve web accessibility refer to commonly used technologies and rules.
The W3C (World Wide Web Consortium) was established to define web standards.
Web standards define HTML as the structure language, CSS as the presentation language, and Script as the behavior language.

FTP (File Transfer Protocol)

kc@example.com (kc kim) — Sun, 03 Apr 2022 18:02:00 +0900

FTP

FTP stands for File Transfer Protocol and is a communication protocol first released in 1971 so computers could exchange files over TCP/IP networks.
It is a service used to transfer files on the Internet. While Telnet is a service used to connect to and use another computer, FTP is used to transfer files to a server computer or to receive files from a server computer onto your own computer.

FTP is still widely used because it can transfer files quickly and reliably. To overcome the disadvantage of having to learn all commands, as with Telnet, dedicated FTP programs appeared and made it convenient to use.

How FTP Works

FTP is a client-server protocol. When the client requests a file, the server provides the requested file. Therefore, two basic channels are required to establish an FTP connection.

One channel is the command channel, which sends commands and basic information such as which files can be accessed.
The other channel is the data channel, which transfers file data between two devices.

To establish a connection, the user must provide login information for the FTP server, and FTP generally uses port 21 as its default communication mode.

FTP Connection Modes

There are two FTP connection modes: active mode and passive mode.

Active mode
- The server takes an active role in approving data requests.
- In active mode, problems caused by firewalls often occur. If a third party attempts to access an unauthorized session, that session is blocked.
Passive mode
- In passive mode, if a third party attempts to access an unauthorized session, the server does not actively maintain the connection.
- In other words, the user sets up both the data channel and the command channel. The server basically only “listens” and does not actively participate, allowing another device to handle most of the work.

FTP Connection Clients

FileZilla: the most widely used free program, available for Windows and macOS.
WinSCP: a free program developed for Windows.

Advantages and Disadvantages of FTP

Advantages
- Multiple files can be transferred at the same time.
- If the connection is lost, the transfer can be restarted.
- Transfer schedules can be reserved.
Disadvantages
- It is vulnerable in terms of security.

What Is SFTP?

FTP was developed in the 1970s, before most of the cybersecurity measures we use today were developed. Because it was not designed as a security protocol, FTP transfers do not encrypt files. As a result, passwords, usernames, and other sensitive data can be read relatively easily through packet capture attacks on data packets.

SFTP (SSH File Transfer Protocol) is a separate protocol that provides highly secure file transfer through a Secure Shell (SSH) data stream. Unlike FTP clients that use port 21, SFTP uses port 22. Because of FTP’s weak security, many people prefer SFTP, which provides security features by default and uses SSH connections.

References

OSI 7 Layers

kc@example.com (kc kim) — Sun, 31 Dec 2017 02:30:37 +0900

OSI Reference Model (OSI 7 Layer)

One term that cannot be left out when discussing standardization in networking is “Open Systems Interconnection” (OSI). OSI is a network model established by the International Organization for Standardization (ISO) in 1977 as part of developing an architecture for communication between heterogeneous computer systems. It shows through what paths and methods data is transmitted and received. By understanding it, you can see how devices such as routers, bridges, switches, and network cards handle data. For people working in networking, OSI can be considered a kind of bible. OSI is called a collection of protocols because each layer that makes up the OSI model plays the role of a protocol. The OSI model consists of seven layers in total.

The fourth layer, the transport layer, through the seventh layer, the application layer, are generally called the upper layers. They are more closely related to application software than to network equipment.

Physical Layer

Defines the physical connection between computers and networks from both mechanical aspects, such as cables and connectors, and electrical aspects, such as voltage and signal modulation. Defines the network topology. Defines the signaling methods used on network media.

Data Link Layer

Defines the protocols that allow computers to exchange messages.

Provides reliable information transmission between two devices through a physical connection.

Network Layer

Defines the path through which packets are transmitted (routing).

Controls the flow of packets sent to nodes on the network and defines how status messages are delivered to network nodes.

Transport Layer

Defines how to locate physical positions on the network, establish connections between nodes, authenticate them, and arrange packets.

It is responsible for collecting packets in the proper order, checking for errors, and sending packets to the session layer.

Session Layer

Provides a conceptual interface to the transport layer for applications.

Allows devices to be identified by name instead of by network address.

Maintains sessions, or connections, between two nodes on the network.

Presentation Layer

Defines how applications enter the network and how data is translated into the form in which it is transmitted and consumed on the network.

If compression and encryption are used, they are also handled by the presentation layer.

Application Layer

The interface used by network-enabled applications to access the network.

Defines network applications.

This is the layer where email and network utilities exist.

It controls application access to the network and notifies applications when network errors occur.

Reasons for Developing Layered Protocols

Layered protocols were developed to separate complex networks by role area so they can be understood logically and more easily. They also provide standardized interfaces between network functions, ensure symmetry among functions performed at each node in the network, and clarify the language used when network designers, administrators, vendors, and users discuss network functions.

OSI 7 Layers

Application Layer (Top Layer)

This layer defines services that support users and application tasks. It manages FTP, databases, remote access (TELNET), email, and similar services.

Presentation Layer

This layer formats data sent from the application layer above it into basic codes such as ASCII or EBCDIC.

Session Layer

This layer manages LAN users when they connect to a server, including connection setup, maintenance, and termination.

Transport Layer

This layer is related to protocols such as TCP and SPX, and is associated with data transmission, error recovery, flow control, and network addressing.

Network Layer

This layer contains the conditions needed to assign destination addresses to data and determine transmission paths. It is the area where various transport protocols such as IP and IPX operate. It manages system connection devices and packets.

Data Link Layer

This layer defines how data packets are formed and transmitted. For example, it plays the role of loading letters onto delivery trucks, sending each truck onto the highway, and ensuring that they arrive safely.

Physical Layer

This layer converts data packets formed by the upper data link layer into electrical or optical signals and transmits and receives them. It can be compared to the trucks and highways needed to deliver letters to different locations.

VPN (Virtual Private Network)

kc@example.com (kc kim) — Mon, 11 Jul 2022 09:48:00 +0900

What Is a VPN (Virtual Private Network)?

A VPN (Virtual Private Network) is a technology that extends a virtual private network so that private networks, which cannot be accessed by third parties, can communicate securely with one another.

As in the following examples, VPNs are used to communicate securely between geographically remote networks.

Securely access a company’s private network from a home private network
Securely access the private network of a Busan branch office from the private network of a Seoul branch office

VPNs are used for the following reasons.

The common way to communicate between geographically separated networks is the internet
However, the internet, which anyone can access, is insecure from a security perspective
Therefore, using a VPN, which extends a virtual private network over the internet, increases communication security
Although the above says that the common way to communicate between geographically separated networks is the internet, there are also several other methods

Communication Methods Between Private Networks

There are three typical methods for communication between private networks.

Provider dedicated line (a dedicated optical cable, exclusively used by the subscriber)
Provider closed network + VPN (a closed network owned by a single provider)
Internet + VPN (a collection of routers from multiple providers, accessible by anyone)

Provider Dedicated Line

A provider dedicated line is a line used exclusively by the subscriber, allowing secure communication between private networks. However, the circuit cost is high.

Provider Closed Network + VPN

A “provider closed network” shares the line among multiple subscribers, so public IP addresses are required to distinguish subscribers, and private networks cannot communicate with one another as they are.

Internet + VPN

Because anyone can use the “internet” line, it has the following two issues.

Public IP addresses are required to distinguish users, and private networks cannot communicate with one another as they are.
Because anyone can use the line, there is a possibility of attack by malicious third parties.

VPN extends private networks over a provider closed network or the internet to provide the functionality of a provider dedicated line while reducing costs.

TCP/UDP (Socket Communication)

kc@example.com (kc kim) — Fri, 08 Jul 2022 18:59:00 +0900

TCP (Transmission Control Protocol)

Provides a connection-oriented service.
Guarantees high reliability.
Establishes connections with 3-way handshaking.
Terminates connections with 4-way handshaking.
Provides data flow control and congestion control.
- Flow control: prevents receiver buffer overflow.
- Congestion control: prevents an excessive increase in the number of packets in the network.
Provides full-duplex, point-to-point service (bidirectional transmission and reception).
Used for cases where reliability is important, such as file exchange.

UDP (User Datagram Protocol)

Provides a connectionless service.
Has low reliability.
The order of data transmission can change.
Does not confirm whether data was received; there is no process like 3-way handshaking.
Has a faster transmission speed than TCP.
Mainly used for streaming, where real-time delivery is important.
Supports 1:1, 1:N, and N:M communication.

E-Mail

kc@example.com (kc kim) — Sun, 03 Apr 2022 18:02:00 +0900

E-Mail

E-mail is a service that lets users send and receive letters using the Internet. It can be divided into web-based e-mail and e-mail that uses POP3.

Web-Based E-Mail

Web-based e-mail includes services such as Gmail and Naver. With this type of mail, users can access the corresponding site on the Internet without a separate program, then manage and receive mail there.

Advantage: Because no separate program is used, it can be used easily when several people share one computer or in places where installing a separate program is difficult.
Disadvantage: Users must use the interface provided by the company and must access that website.

POP3

With a POP3 server, users can send and receive mail using a POP3-supported e-mail client such as Outlook Express.

Advantage: Because it simply delivers letters, any program that supports POP3 can be used, so users can choose a program that is convenient for them.
Disadvantage: A program must be installed to check e-mail contents.

What Is Network TCP/IP?

kc@example.com (kc kim) — Tue, 10 Jan 2023 11:21:21 +0900

What Is TCP/IP?

TCP/IP is a combination of TCP (Transmission Control Protocol) and IP (Internet Protocol), and it can be considered one of the most important technical elements for supporting computer networks.

In general, “TCP/IP” is used as a collective term for the communication technologies that operate computer networks and the internet. It includes not only TCP and IP, but also various protocols such as ICMP, TCP, and HTTP. In any case, you can think of it as the set of protocols needed to send something over a network.

What Is a Protocol?

Here, a protocol means a “communication rule.” Data is transmitted together with various elements.

Data A: the data that starts first
Data B: the next data
Data C: the data after that

If the order in which data is sent is not determined in this way, it becomes impossible to know what was actually sent.

For example, if you know that data such as “recipient,” “where it came from (your own address),” and “data contents” is sent in order, the receiving side can also identify the data by recognizing it as “recipient,” “where it came from (your own address),” and “data contents.”

That is why protocols are essential.

In short, you can think of TCP/IP as the communication rules for a network.

Through What Interaction Is Data Transmitted?

For example, suppose you are viewing a site on the internet. A request like the following is sent through the network.

The browser creates a request message.
The operating system’s TCP/IP processing software passes the data to a place called the transport layer.
The TCP protocol in the transport layer attaches a TCP header, meaning that this kind of data is added to the beginning of the data.
Next, the data is sent to a place called the network layer, where an IP header, containing data such as the destination, is attached.
Then header information such as a MAC header is added on top of that, and the data is sent from the LAN adapter.

This is difficult. It may be a little hard to understand, but you can think of it as sending data after attaching various pieces of information to the beginning of the data as needed.

IP (Internet Protocol)

kc@example.com (kc kim) — Fri, 08 Jul 2022 18:48:00 +0900

What Is IP?

Internet Protocol
A protocol for delivering data on the Internet.
It is a protocol that delivers packets through the Internet network.
It has the characteristics of being connectionless and unreliable.
- Connectionless means it does not predetermine the path for sending packets.
- Unreliable means it does not guarantee complete packet delivery.

Difference Between Public IP and Private IP

Public IP
- A globally unique IP address provided by an ISP (Internet service provider).
- Because it is exposed externally, it can be accessed by other devices connected to the Internet.
- Therefore, security settings such as a firewall must be configured.
Private IP
- An IP address used inside a specific network.
- Because the shortage of IPv4 addresses makes it impossible for every network to use public IP addresses, it is a virtual address assigned through a router inside a network.
- It cannot be accessed from outside without separate configuration.

IP Address

kc@example.com (kc kim) — Tue, 20 Dec 2022 11:41:27 +0900

IP Address

An IP address is the address of a computer on the Internet. Every device connected to the Internet must be assigned an IP address.

How Is Data Transmitted on the Internet?

For example, when you “send mail,” “send a message through a messenger,” or “buy a product from an online store,” how is data sent and received?

In fact, it is similar to sending a parcel.

For a parcel to arrive safely, information about the “sender address” and “recipient address” is required.

Flow of parcel delivery:

First, it is gathered at the office closest to the “sender address.” (In the figure above, the Seoul office)
Next, the “recipient address” is checked, and the parcel is sent to the office closest to that address. (In the figure above, the Busan office)
Finally, it arrives at the “recipient address” from the office.

Now let’s look at how data is sent and received on the Internet.

To deliver a parcel to another person, the “sender address” and the “destination address” were required.

In a network, these addresses correspond to “IP addresses.” For a computer (network device) to connect to a network, it must be assigned a unique IP address that does not overlap with others.

The flow of data transmission and reception by a computer (network device) is as follows.

First, data is sent to a network device called a router near the “source IP address” (a role similar to a parcel office).
Then the router checks the “destination IP address” and sends the data to a router near the “destination IP address.”
Finally, the data is delivered from the router to the computer (network device) assigned the “source IP address.”

In this way, the structure of data transmission and reception on the Internet is almost the same as parcel delivery. Work that people perform is automatically performed by using network devices (routers).

How IP Addresses Work

Who Manages IP Addresses?

An IP address is the address of a computer (network device) on the Internet. Therefore, IP addresses are assigned so that they do not overlap.

The organization that manages “IP addresses,” which are indispensable for communication on the Internet, is a nonprofit corporation called ICANN (The Internet Corporation for Assigned Names and Numbers).

However, ICANN does not manage all information in the world directly. Under ICANN are RIRs (Regional Internet Registries) that manage resources by region such as North America, Asia, and Europe. Under them are NIRs (National Internet Registries) that manage resources by country, and finally LIRs (Local Internet Registries) such as Internet service providers (ISPs).

Difference Between Private IP Addresses and Public IP Addresses

There are two types of IP addresses: “private IP addresses” and “public IP addresses.”

A public IP address is an IP address required to connect to the Internet, while a private IP address is used only inside an organization’s network (private network), such as a home or company.

Private IP Address

A private IP address is an IP address used in an organization’s network (private network), such as a home or company. Addresses are assigned so they do not overlap within the organization.

The following is an example of private IP addresses.

In the figure above, the left side is the “private network,” the right side is the “Internet,” and there is a broadband router between the “private network” and the “Internet.” This is a common configuration when using an Internet service provider (ISP).

Computers in the private network are assigned private IP addresses.

Using private IP addresses, computers can communicate with other computers inside the private network.

However, to communicate with computers outside the private network, a public IP address is required.

Private IP addresses cannot communicate on the Internet.

Public IP Address

A public IP address is an Internet address required for computers and communication devices to communicate over the Internet.

Therefore, a router (broadband router) connects to the Internet by converting private IP addresses into public IP addresses. The method of converting between private IP addresses and public IP addresses is called NAT (Network Address Translation).

Why Are Private IP Addresses Necessary?

Private IP addresses appeared because of the IPv4 “IP address exhaustion problem.” IP address exhaustion is the problem of not having enough IP addresses.

An IP address is an address for communicating on the Internet. Therefore, an IP address must be unique and must not overlap.

Based on the idea that not every computer needs to access the Internet, a mechanism was created to assign a specific range of IP addresses inside an organization as private IP addresses.

Unlike global IP addresses, which must be assigned so they do not overlap anywhere in the world, private IP addresses only need to be unique within an organization (private network).

In the figure above, computers in “private network A” and “private network B” are assigned the same private IP address, but this is not a problem because the organizations (private networks) are different.

Therefore, private IP addresses are a mechanism for conserving IP addresses.

Private IP addresses also have security advantages. To communicate on the Internet, traffic must pass through a router (broadband router, default gateway). Because this router has security functions, security can be strengthened through the router.

IP Address Definition

Classful Method

An IP address consists of two parts: a network part and a host part.

The network part indicates which network it belongs to, and the host part is the address assigned to a host in that network.

Traditional IP addresses used the “classful method,” so the network part and host part were fixed.

Class	Address range	Network part range	Host part range	Number of assignable hosts
Class A	0.0.0.0 ~ 127.255.255.255	First 8 bits	24 bits	16,777,214
Class B	128.0.0.0 ~ 191.255.255.255	First 16 bits	16 bits	65,534
Class C	192.0.0.0 ~ 223.255.255.255	First 24 bits	8 bits	254

Because of this structure, for example, if a company that needs 100 IP addresses uses “Class C,” Class C provides 254 assignable hosts, so 154 IP addresses are wasted after excluding the required 100 addresses (254 - 100 = 154).

This led to the “classless method.” In the classless method, a subnet mask is used to make it possible to freely change the boundary between the “network part” and the “host part.”

Classless Method

The classless method is a method that uses a subnet mask to make it possible to freely change the boundary between the “network part” and the “host part.”

A subnet mask is written in decimal as “255.255.255.0,” as in the example above, and in binary as “1” and “0.”
The parts of the subnet mask that are “1” are the “network part,” and the parts that are “0” are the host part.

For example, suppose a company that needs 100 IP addresses is assigned the Class C network “192.168.1.0,” as before. (The subnet mask is 255.255.255.0.)
In this case, 254 IP addresses can be used by computers (192.168.1.1 ~ 192.168.1.254).
As a result, 154 IP addresses are wasted (254 - 100 = 154).

Now move the boundary between the “network part” and the “host part” one position to the right.

Then the subnet mask changes from “255.255.255.0” to “255.255.255.128.” In this case, 126 IP addresses can be used by computers (192.168.1.1 ~ 192.168.1.126).
As a result, only 26 IP addresses are wasted (126 - 100 = 26). If you consider them spare addresses, this is not really waste.

In this way, the “classless method” uses a subnet mask to freely change the boundary between the “network part” and the “host part” and adjust the number of IP addresses assigned to hosts.

Difference Between IPv4 and IPv6

The major versions of IP addresses are IPv4 and IPv6. IPv6 appeared because IPv4, the major IP address version, has an “IP address exhaustion problem.”

IPv4 provides about 2 to the 32nd power usable IP addresses (about 4.3 billion = 4.3 x 10^9), while IPv6 provides about 2 to the 128th power usable IP addresses (about 340 undecillion = 3.4 x 10^38).

IPv6 addresses are expressed in 128 bits, separated by colons (:) every 16 bits, and written in hexadecimal.

An example IPv6 notation is as follows.

201a:30b8:cd01:0132:2f8a:1f30:1c02:10aa

How to Check an IP Address

Checking an IP Address on Windows

To check your IP address on Windows, start Command Prompt and use the ipconfig command.

C:\>ipconfig

Windows IP Configuration


Ethernet adapter vEthernet (Default Switch):

   Connection-specific DNS Suffix  . :
   Link-local IPv6 Address . . . . . : fe80::1a8:2f:14f:e65b%42
   IPv4 Address. . . . . . . . . . . : 172.25.32.1
   Subnet Mask . . . . . . . . . . . : 255.255.240.0
   Default Gateway . . . . . . . . . :

Wireless LAN adapter Local Area Connection* 9:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :

Wireless LAN adapter Local Area Connection* 10:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :

Wireless LAN adapter Wi-Fi:

   Connection-specific DNS Suffix  . :
   Link-local IPv6 Address . . . . . : fe80::6107:be9e:21cc:5dd8%14
   IPv4 Address. . . . . . . . . . . : 192.168.0.11 ----------> "192.168.0.11" is the IP address.
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 192.168.0.1

Ethernet adapter Bluetooth Network Connection:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :

When connecting to the Internet through a broadband router or similar device, most IP addresses displayed by the ipconfig command are “private IP addresses.”
If you are not using a broadband router or similar device, a “public IP address” is displayed.

Checking an IP Address on macOS

On macOS, you can check it with the terminal command ifconfig | grep inet.

% ifconfig | grep inet
	inet 127.0.0.1 netmask 0xff000000
	inet6 ::1 prefixlen 128
	inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
	inet6 fe80::a08e:1eff:fe6e:5cd7%anpi1 prefixlen 64 scopeid 0x4
	inet6 fe80::a08e:1eff:fe6e:5cd9%anpi3 prefixlen 64 scopeid 0x5
	inet6 fe80::a08e:1eff:fe6e:5cd8%anpi2 prefixlen 64 scopeid 0x6
	inet6 fe80::a08e:1eff:fe6e:5cd6%anpi0 prefixlen 64 scopeid 0x7
	inet6 fe80::bc76:eff:fe35:99b8%ap1 prefixlen 64 scopeid 0x10
	inet 192.168.0.3 netmask 0xffffff00 broadcast 192.168.0.255 ----------> "192.168.0.3" is the IP address.
	inet6 fe80::c4a:ec1f:c825:f3ff%en1 prefixlen 64 secured scopeid 0x11
	inet6 fe80::349c:a6ff:fe51:afdd%awdl0 prefixlen 64 scopeid 0x14
	inet6 fe80::349c:a6ff:fe51:afdd%llw0 prefixlen 64 scopeid 0x15
	inet6 fe80::9e50:c1a4:1b3c:b91%utun0 prefixlen 64 scopeid 0x16
	inet6 fe80::1d5b:41a9:12e2:c38e%utun1 prefixlen 64 scopeid 0x17
	inet6 fe80::ce81:b1c:bd2c:69e%utun2 prefixlen 64 scopeid 0x18

What Is a Network Port?

kc@example.com (kc kim) — Tue, 10 Jan 2023 11:40:06 +0900

What Is a Port?

A Port Is a Harbor

A port originally means a harbor. A harbor is the space ships need to enter a town, so it acts as the entrance between the sea and the town.

Ports used in IT are similar. At a harbor, it is decided where each item of cargo will be unloaded. Numbering those places makes the process clearer. In the same way, if the entire Internet is the sea and a computer is a town, the place where data arriving from the Internet is unloaded is defined. Data for viewing web pages goes to one harbor number, mail goes to another harbor number, and so on.

This corresponds to a “port.”

When communication is actually performed with TCP or UDP, communication is not handled per computer, but per “program,” that is, per process or thread. Therefore, data must be delivered correctly between processes. At this time, a “port number” is assigned to the communicating process. Processes and threads use this port number to determine which application should communicate with which application.

Building Analogy

Ports are easier to understand if you compare them to a building.

Network communication requires a protocol, an IP address, and a port number.

If an IP address is like the address of a building, a port number is like a “room number.” In a computer network, protocol, IP address, and port number can be rephrased as: “By what method (protocol), to which building (computer, IP), and to which room (port number) should communication be sent?”

Because port numbers are important for communication, opening a port is like opening the door to a specific room. Unless the machine is a server providing a service, unnecessary ports should generally be configured so they remain closed.

More Precisely, a Port Is a Service Window

A port is “a number used to identify a server application” and “a service window provided by a server.” For example, the HTTP protocol commonly uses port 80.

When a port is open, the service window is available. From the server’s point of view, it must keep the window open to provide a service to others. For example, to provide a typical web service over HTTP, port 80 must be opened. This is the work of opening the window required for HTTP communication.

Then what happens if the port remains closed? The server’s window is abandoned. In other words, the service is closed for business and cannot be provided. Therefore, port opening means “opening the window required for each service.”

Common Port Numbers and Protocols

Representative examples of “which protocol operates on which port number” are as follows.

TCP 20: FTP (data)
TCP 21: FTP (control)
TCP 22: SSH
TCP 23: Telnet
TCP 25: SMTP
UDP 53: DNS
UDP 67: DHCP (server)
UDP 68: DHCP (client)
TCP 80: HTTP
TCP 110: POP3
UDP 123: NTP
TCP 443: HTTPS
Well-known port numbers: 0-1023

An example involving ports, IP addresses, and protocols is a web server.

On a server published at a specific IP address, an application that follows the HTTP protocol, such as Apache, waits for communication from clients on port 80 and sends web page information in response to requests.

Other services similarly provide services by opening an arbitrary port or a well-known port for an application that implements a protocol on a specific server.

Well-Known Port Numbers

Port numbers actually range from 0 to 65535, but 0 to 1023 are called “well-known port numbers.” As the name suggests, they are well-known port numbers. These numbers are managed by an organization called IANA.

They are reserved port numbers used by major services and protocols in TCP/IP and UDP communication.

Just as 80 is HTTP and 22 is SSH, major services generally have standard port numbers.

It is possible to edit the configuration file for each service and change the listening port number. In some cases, the port number is intentionally changed for security reasons, but in general, use the well-known port numbers.

Registered Port Numbers

Port numbers 1024 to 49151 are also managed by IANA.

They are a group of ports intended for use by specific applications and similar software. IANA accepts registrations and publishes them.

Other Port Numbers

Port numbers 49152 to 65535 can be used freely by users. They do not need to be registered with IANA.

For port numbers, there is no absolute rule for “which service should run on which port,” so they can also be managed, compared, and used arbitrarily.

Port Opening and Security

Leaving a service window open is like leaving a window unlocked. Naturally, this makes the system more likely to be targeted by thieves trying to steal information, that is, crackers.

Because this state is very undesirable, ports must not be opened carelessly. Security measures should be established properly, and only the minimum necessary ports should be opened.

Modern computers have very high processing power, and more applications can remain resident as servers even on personal machines. Applications that function as servers often respond not only to local information but also to information from outside. These server applications inevitably open ports in order to provide services.

However, if a server application has a security vulnerability, in the worst case the server may be taken over or become unable to provide services. Applications that function as servers must be reviewed carefully. In addition, for computers operating as servers, it is necessary to check “which ports are open” and “whether any unintended ports are open.” Before actual operation, it is important to perform a penetration test, actually run a port scan, and understand the current state.

DNS Server

kc@example.com (kc kim) — Fri, 30 Dec 2022 20:21:01 +0900

What Is DNS (Domain Name System)?

DNS stands for Domain Name System. It is a system that manages a mapping table between “IP addresses,” which are Internet addresses that computers can easily understand, and “domains,” which are Internet addresses that people can easily understand.
For example, DNS converts a human-readable domain name such as www.devkuma.com into a computer-readable IP address such as 185.199.109.153.

DNS Server

A DNS server is a server that implements name resolution. Domain name resolution is a system that manages and converts the mapping table between “IP addresses” and “domains,” which are addresses on the Internet.

A computer that provides a service or function is called a “server,” and a computer that uses that service or function is called a “client.”

A DNS server is a server that implements the function of converting a “domain” into an “IP address” or an “IP address” into a “domain” (name resolution).

Why Is a DNS Server Necessary?

To communicate on the Internet, an Internet address called an “IP address” is required.

Therefore, when browsing a website or sending email, something easy for people to understand, called a “domain,” must be converted into an “IP address.”

For example, the URL of this site is “https://devkuma.com.” The domain name is “devkuma.com.” To view this site, communication is performed by converting the domain “devkuma.com” into an “IP address.”

The same applies when you want to send mail to the destination mail address “kimkc@devkuma.com.” The part after “@” is the domain “devkuma.com,” and communication is performed by converting that domain into an “IP address.”

If the value is converted anyway, you might think that “domains” are unnecessary from the beginning. However, without domains, addresses would be written as below, making it difficult to understand what site they refer to.

https://185.199.109.153.com
https://127.0.0.1.net
https://192.168.10.1.co.kr

The purpose of DNS is to make addresses easier to read by representing them with characters instead of IP address numbers.

How DNS Servers Work

There are many kinds of DNS servers. The first server a user’s computer queries is a “DNS cache server.”

A “DNS cache server” stores query results for a certain period. If the same request arrives within that period, it responds by itself without newly requesting an external DNS server.

At the top of the hierarchical DNS servers (three layers in this example) is the “root server.” The “root server” manages the IP addresses of DNS servers for top-level domains such as “kr,” “com,” and “net.”

Below the top-level domain DNS servers, it manages the IP addresses of DNS servers that manage domains such as “xx.com.” Finally, the flow is to obtain the IP address from the DNS server for “xx.com” that manages the IP address of “devkuma.com.”

In this way, DNS does not manage everything with a single server. Instead, it distributes management by arranging servers hierarchically and managing the mapping between “domain names” and “IP addresses.”

Domain Name System Organizations

Domain names are usually written in the order “host.server domain (organization name).subdomain (organization type).top-level domain.” For example, the Blue House domain is “www.president.go.kr.”

Top-level domains represent organizations or countries by abbreviation. In the case of the United States, the country name is often omitted and domains frequently end with one of the domains in the left side of the table below. For example, the White House domain name is “www.whitehouse.gov.”

Types of Top-Level Domains

Domain	Organization	Domain	Country
edu	Educational institution	kr	Korea, South
com	Company/business	jp	Japan
gov	Government agency	cn	China
int	International organization	ca	Canada
org	Nonprofit public organization	de	Germany
net	Network-related organization	uk	United Kingdom
mil	U.S. Department of Defense-related organization	fr	France

Learn more about country code top-level domains

A subdomain is the area between the top-level domain and the host name. As shown in the table below, it represents the organization’s name and type. Subdomains that indicate the organization’s type include ac, re, co, go, and ne, representing schools, research institutes, companies, government agencies, and network-related organizations, respectively.

Types of Subdomains

Subdomain	Organization	Domain	Example
ac	Academy	Educational/academic institution	https://www.snu.ac.kr
re	Research	Research institute	https://etri.re.kr
co	Company	Company	https://samsung.co.kr/
go	Government	Government agency	http://www.president.go.kr/
ne	Nonprofit public organization	Network or gateway

Learn more about .kr

DNS Records

A DNS record (or zone file) is an instruction on an authoritative DNS server that provides information about the IP address associated with a domain and how requests for that domain should be handled. These records consist of a set of text files called DNS syntax. DNS syntax is a string used as a command that tells the DNS server what work to perform. Also, every DNS record has a “TTL,” which stands for time-to-live and indicates how often DNS servers refresh that record.

A

An IPv4 address record is used to map a host name to an IPv4 address.

AAAA

An IPv6 address record is used to map a host name to an IPv6 address.

CNAME

A canonical name record is an alias that points to another DNS record.

NS

A name server record points to an authoritative name server.

SOA

A start of authority record contains authoritative information about a DNS zone.

MX

Sends email to an email server.

TXT

A text record contains arbitrary text.
Administrators can store text notes in the record.
This record is often used for email security.

RSS

kc@example.com (kc kim) — Sun, 27 Nov 2022 17:59:00 +0900

Overview

RSS is a format for distributing a brief summary of a website’s title or contents. It is mainly used to publish update information.

It is generally used to distribute blog update information, but it is also often used by news sites and TV schedule sites to distribute new article or program information, and by companies to distribute product information.

The term RSS is also referred to by the following names.

RDF Site Summary
- RDF stands for Resource Description Framework, meaning a tool for writing metadata.
- In other words, it is an agreement to “write this kind of information in this kind of form.”
Really Simple Syndication
- Very simple distribution
Rich Site Summary
- Rich site summary information

Emergence and Growth of RSS

The first development of RSS began from a concept at NetCenter, Netscape’s Internet portal site. Netscape was famous for its browser, and RSS was devised to provide articles from well-known newspapers easily.

Looking briefly at Netscape’s development history, the RSS format started from MCF (Meta Content Framework) in 1995, passed through the development of RDF (Resource Description Framework) and CDF (Channel Definition Format), and appeared as RSS (RDF Site Summary). However, Netscape abandoned further development after RSS 0.9. After that, two development groups formed and continued development, which is why RSS came to have two names.

RSS Standards

RSS (RDF Site Summary) 1.0 from the RSS-DEV Working Group and RSS (Really Simple Syndication) 2.0 from UserLand are industry standards.
The two specifications have slight functional differences, but UserLand’s 2.0 provides more detailed functions.

RSS 1.*: specifications based on RDF
- RSS 0.9
- RSS 1.0
- RSS 1.1 (January 2005)
RSS 2.0: specifications not based on RDF
- RSS 0.91
- RSS 0.92
- RSS 0.93
- RSS 0.94
- RSS 2.01

Other formats also appeared, such as Media RSS announced by Yahoo in December 2004, and around the end of 2004, methods of extending RSS formats and applying them to podcasting also appeared.

With the spread of RSS, recognition of the importance of content syndication, new functions, and the need for standardization grew. Many discussions and efforts for content syndication standardization took place, but it became difficult to unify and standardize RSS specifications, and this led to a new standardization project named Atom.

RSS continues to evolve. Although there are slight differences in methods, RSS is becoming established on the Internet because it develops based on the basic philosophy of being “open.”

Benefits of RSS Services

RSS has the following advantages.

Selective subscription
- Users can choose channels that exactly match the topics they want.
Fast subscription
- Multiple channel sources can be accessed at the same time.
History management
- Past records from various channels can be stored.
Easy automated content integration
- syndication / aggregation
Content reusability
- Structured XML data can be easily transformed and processed.
Change in communication method
- From one-to-one to one-to-many simultaneous access

RSS 2.0 Specification

0. `<rss>` Element

<rss version="2.0">
</rss>

A document complying with RSS 2.0 must have an element named rss as the document element.
The <rss> element must have the version attribute, and its value is always "2.0".
The <rss> element has the <channel> element as a child.

1. `<channel>` Element

The channel corresponds to a website. It has three required child elements and fifteen optional child elements.

Required Channel Elements

<title>: the name of the channel. Usually the same as the website name.
<link>: the address of the website that supplies this RSS.
<description>: promotional text that introduces or describes this channel (website).

Optional Channel Elements

<language>: specifies the main language of the channel according to ISO 639.
<copyright>: contains copyright text.
<managingEditor>: e-mail address of the post editor.
<webMaster>: e-mail address of the site administrator.
<pubDate>: publication date and time of this RSS, in RFC 822 format.
<lastBuildDate>: last modification time of posts in this channel, also in RFC 822 format.
<category>: topic or category of the channel.
<generator>: software used to publish this RSS.
<docs>: address where this RSS can be received.
<cloud>: node specifying RSS Cloud features such as real-time notification and opinion exchange.
<ttl>: limits how long the feed can be cached before it is fetched again.
<image>: specifies an image such as a logo representing the channel.
<textInput>: specifies a text box that can be displayed in the channel, often for search or feedback.
<skipHours>: specifies hours from 0 to 23 when RSS collectors should not fetch.
<skipDays>: specifies days when RSS collectors should not fetch.

`<item>` Element

The <item> element represents a post in the channel. It has required elements and optional elements.

<title>: title of the post.
<link>: URL of the post.
<description>: body of the post or part of the body.
<author>: e-mail address of the post author.
<category>: category to which the post belongs.
<comments>: address where comments for the post can be read.
<enclosure>: node for attached files, with url, length, and type attributes.
<guid>: a unique string that can programmatically identify only this post.
<pubDate>: date when the post was registered.
<source>: if the post came from another RSS feed, indicates that feed’s name and address.

Example RSS Written in RSS 2.0

<?xml version="1.0" encoding="utf-8"?>
<rss version="2.0">
   <channel>
      <title>Liftoff News</title>
      <link>http://liftoff.msfc.nasa.gov/</link>
      <description>Liftoff to Space Exploration.</description>
      <language>en-us</language>
      <pubDate>Tue, 10 Jun 2003 04:00:00 GMT</pubDate>
      <lastBuildDate>Tue, 10 Jun 2003 09:41:01 GMT</lastBuildDate>
      <docs>http://blogs.law.harvard.edu/tech/rss</docs>
      <generator>Weblog Editor 2.0</generator>
      <managingEditor>editor@example.com</managingEditor>
      <webMaster>webmaster@example.com</webMaster>
      <item>
         <title>Star City</title>
         <link>http://liftoff.msfc.nasa.gov/news/2003/news-starcity.asp</link>
         <description>How do Americans get ready to work with Russians aboard the International Space Station?</description>
         <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
         <guid>http://liftoff.msfc.nasa.gov/2003/06/03.html#item573</guid>
      </item>
   </channel>
</rss>

References

CDN (Content Delivery Network)

kc@example.com (kc kim) — Wed, 30 Mar 2022 08:52:00 +0900

CDN

Content Delivery Network or Content Distribution Network.
A CDN (Content Delivery Network) is a platform made up of densely distributed servers that minimizes delays in loading web page content by reducing the physical distance between servers and users.
It is a technology that provides content more quickly and reliably to users who are geographically and physically distant.
It is a service for improving slow response speeds and download times for users.
It provides content by caching content from a remote origin server through caching servers inside the CDN.
This allows users around the world to view the same high-quality content without slowing loading times.
When a request for content occurs, a CDN maps the request to the server closest to the user and handles it with a cached (previously stored) version of the requested file. If the server cannot find the file, it searches for the content on another server in the CDN platform and then sends the response to the end user.

Image source: https://en.wikipedia.org/wiki/Content_delivery_network

Homepage

kc@example.com (kc kim) — Wed, 30 Nov 2022 21:08:00 +0900

It is often used with the same meaning as a website. In other words, it is used as a term that indicates a place where data or files are gathered in one place on the WWW (World Wide Web), and it has abbreviations such as HP and homepi. However, this is incorrect usage. Originally, a homepage referred to the start screen (website) displayed first when a web browser is launched. In Korea, website and homepage are often used interchangeably with the same meaning, but the two are actually different.

Website
- A collection of web pages for content or services stored on a web server to be provided on the Internet.
- It means a collection of web pages belonging to one domain.
Homepage
- The page shown first on every website, or the first page.
- The page connected first when visiting the website above is the homepage.

Most web browsers have a “Home” button, and clicking that button performs the action of returning to the start screen. In other words, homepage originally meant “the page displayed as the home screen (start screen).”

In the early days of the Internet, many websites used a Home button as the button for returning to the start page, so the meaning changed to “website start page = homepage.” In addition, beginner books for studying HTML often used homepage creation examples with titles such as “Welcome to OO’s homepage!” This spread widely. Web pages produced everywhere like mushrooms after rain all used the word “homepage,” and magazines and media also continued to use the word incorrectly. As a result, among the general public, “homepage” became a more familiar term than “website.”

References

Understanding Network Terms with Pictures

Proxy

kc@example.com (kc kim) — Fri, 08 Jul 2022 18:56:00 +0900

Proxy Server

A proxy is a computer system or application that allows a client to connect indirectly to another network service through it.
Communication performed on behalf of another party as an intermediary between a server and a client is called a “proxy,” and the component that performs this intermediary function is called a proxy server.

Types of Proxies

Forward Proxy

In general, when people say proxy, they usually mean a forward proxy.
It is a method where a client does not request a server directly, but first sends the request through a proxy server.
- To make it easier to understand, when a company intranet makes a request to a server on the Internet, it first calls a proxy server. This method is called a forward proxy.
It hides who the client is from the server.
- Because the IP address received by the server is the IP address of the forward proxy server, the server cannot know who the client is.
In the diagram below, if the proxy server is called first, it is a forward proxy.
Example: VPN

Reverse Proxy

A reverse proxy can be understood as the opposite direction from a forward proxy.
When a client calls a server, it calls the reverse proxy, and the proxy server sends the request and delivers the received response to the client.
In an internal intranet, this is a method where a client on the Internet requests the reverse proxy server in order to call a server and receives the response.
A reverse proxy hides who the server is.
- Because the client calls the reverse proxy server first, it cannot know the IP address of the actual server.
In the diagram below, the method where a client on the Internet calls a proxy server and then calls a server in the internal network is a reverse proxy.
Example: Gateway

Benefits of Proxies

By using a proxy server, which is an intermediary server, instead of direct communication between client and server, you can gain several advantages such as security, performance, and traffic distribution.
A proxy can cache content requested from a server and return an immediate response for the same request, which can save time and resource usage.
Security
- Using a proxy server makes it possible to hide the IP address of the client or server. By hiding the actual server or client IP address and exposing only the proxy server’s IP address, it can help prepare against hacking.
Performance
- Using a proxy server can improve performance through caching and distribution.
- Caching reuses frequently used identical requests by storing them.
- The proxy server responds instead of calling the actual server again, reducing server resource usage.
Traffic distribution
- Some proxy servers also provide load balancing, so if there are multiple distributed servers, server traffic can be distributed.
- It is also possible to configure which server is called for each endpoint (URL), distributing server traffic by role.
Others
- A single proxy server can handle HTTPS certificate management, while the servers running behind it provide service over HTTP. This has the advantage that certificates do not need to be managed on every server.

Difference Between Forward and Redirect

kc@example.com (kc kim) — Thu, 07 Apr 2022 01:19:00 +0900

Forward Method

Forward only moves pages at the Web Container level. In practice, the web browser cannot tell that it has moved to another page. Therefore, the web browser displays the URL that was first called, and the URL information of the page that was moved to cannot be checked.
The currently running page and the page called by forward share the Request object and Response object.

Statement used when moving from a Servlet to a JSP page

RequestDispatcher rd = request.getRequestDispatcher("test01.jsp");
rd.forward(request, response);

Redirect Method

When a Redirect command enters the Web Container, it instructs the web browser to move to another page. Then the web browser changes the URL to the instructed address and moves to that address. It moves to an address in another web container.
On the new page, new Request and Response objects are created.

Statement used when moving from a Servlet to a JSP page

response.sendRedirect("test02.jsp");

Source

Difference between redirect and forward

Port Forwarding

kc@example.com (kc kim) — Thu, 08 Sep 2022 08:16:00 +0900

What Is Port Forwarding?

Port forwarding is a feature that allows traffic entering through a router’s port to reach a specific port on a device connected to that router.

For example, if a computer and a phone are connected to a router and you simply enter the router address plus a port number, the router does not know which device’s port the request should go to. Port forwarding can therefore be understood as forwarding a router’s port to the port of a device connected to it. It can also be described as assigning a port number on the router to a specific device and that device’s port. For example, connecting to port 180 on the router can lead to port 5000 on the phone, while connecting to port 280 can lead to port 5000 on the computer. The work of assigning values such as 180 and 280 is port forwarding.

Concept and Features of a Load Balancer

kc@example.com (kc kim) — Thu, 08 Sep 2022 08:16:00 +0900

What Is a Load Balancer?

A load balancer is a system that distributes server load.

Types of Load Balancers

L4: Port-Based

Distributes load using information from layer 4 and below.
In particular, it distributes traffic using MAC (layer 2), IP (layer 3), and port (layer 4) information.

L7: URL-Based

Distributes load using application layer information.
It can inspect packet contents, distribute traffic, and filter abnormal traffic such as DDoS attacks.

Infrastructure

kc@example.com (kc kim) — Tue, 20 Dec 2022 17:28:16 +0900

What Is Infrastructure?

Infra is an abbreviation of Infrastructure and refers to the foundational or base part of something. It is an English word that means “foundation” or “underlying structure.”

In general, infrastructure refers to “the foundation of industry and social life.” Facilities that serve as the basis for industrial activities and social life are called “infrastructure.” Specifically, it refers to things that form the foundation of daily life, such as roads, railways, water and sewage systems, power plants, power grids, and communication facilities.

Public facilities that form the foundation of everyday life are called “infrastructure.”

What Is Infrastructure in IT?

In the IT field, “infrastructure” refers to building the foundation for safely operating systems, such as designing, building, and operating servers and networks that run applications.

Infrastructure Engineer

An engineer who builds infrastructure environments is called an infrastructure engineer. Infrastructure engineers build the foundation needed to run applications created by programmers or system engineers, such as server construction and network construction.

In contrast to handling the visible front side such as apps, web pages, and other front-end areas, they handle back-end work behind servers and operating systems.

People who work on infrastructure foundations are called infrastructure engineers. Types of infrastructure engineers include the following.

Network engineer
Security engineer
Server engineer
Database engineer

Work of an Infrastructure Engineer

Infrastructure engineers design and build IT infrastructure, and perform operations and maintenance so it works without problems.

Requirements definition
- Listen to the customer’s requests and decide what kind of IT infrastructure to create.
Design
- Decide what devices to use and what configuration and setting values to set.
- Also consider what rules will be used to operate it after it starts running.
Construction
- Procure equipment and actually perform construction tasks such as connecting devices.
Testing
- Check whether it operates without problems according to the design document.
Maintenance
- Monitor whether it is operating normally and resolve problems when errors occur.

Server

kc@example.com (kc kim) — Sat, 31 Dec 2022 02:18:16 +0900

What Is a Server?

Simply put, a server is a data storage location on a network.
The word server has the meaning of “a place that provides.” A computer that provides corresponding data in response to user demands (requests) through a network such as the Internet is called a “server.”
Servers are used for all Internet-related services, such as browsing web pages, mail, SNS, and online shopping.

Server and Client

In a network, the side that provides data is called the server, and the side that uses data is called the client.
When browsing a website or sending mail, data files such as video files or HTML files are returned from the server in response to requests from the client.
The main role of a server is to send processed files or store data according to client requests.

Upload and Download

Providing information from a client to a server is called “upload.”
(Example) Posting a video to a video sharing site, uploading a photo to SNS

On the other hand, receiving a file from the server that provides it is called “download.”
(Example) Bringing images and videos published on the Internet into a computer

How Servers and Networks Work

To help visualize the role of a server, let’s use the flow from creating a homepage to having visitors view it as an example.

Upload Homepage Data to a Server

Simply having homepage data on your own PC is not enough to show it to Internet users.
To make the homepage viewable, the homepage data must be stored on a server.

When a User Enters a URL in a Browser, a Request Reaches the Server

When an Internet user enters the homepage URL into a browser such as Google Chrome and presses Enter, a request is sent to the server.

URL
- A string that indicates the location of a web page; it is also called an address.
- The URL of the top page of the current site is “https://www.devkuma.com.”
Browser
- Software for viewing homepages (websites) on a PC or smartphone through the Internet.
- Google Chrome, Safari, and Internet Explorer are well-known examples.

The Server Provides Data to the User’s PC or Smartphone According to the Request

The server retrieves the page data that corresponds to the URL and provides files such as HTML and images to the user’s PC, smartphone, or other device.

In addition, to make the site accessible through search engines such as Google or Naver, it is necessary not only to upload homepage data to the server, but also to index (register) it.
Simply uploading homepage data to the server does not make the site appear in searches on Google, Naver, and similar services.
To allow users to access the site through search engines instead of directly entering the URL in the browser, it is essential to index the homepage data in the search engine’s database.

Server Types

Servers as computers can be broadly divided into two types: “physical servers” and “virtual servers.”

Physical Server

A physically existing single server can be divided into two types: “shared server” and “dedicated server.”

Dedicated Server

A dedicated server is a server used exclusively by one user. It has the advantage of a high degree of freedom, such as not being affected by other users and being able to customize the OS, but operating it with proper customization requires a certain level of expertise.

Shared Server

This is a rental server where multiple users share and use a single physical server. It has the advantages of lower cost and less maintenance effort, but because multiple users share it, customization is not easy. Processing can be affected by other users’ usage, and if the server fails, many users can be affected.

Virtual Server

A virtual server is a server that runs multiple operating systems on one physical server and operates them as multiple servers. It can be divided into two types: “VPS” and “cloud server.”

VPS Server

VPS stands for Virtual Private Server and is also called a virtual dedicated server.
Like a shared server, it shares one physical server, but because each environment is built virtually and separately, it has the advantage of being less affected by other users and offers high customizability.

Cloud Server

A cloud server is a server created in a cloud environment. Like a VPS, it builds a virtual dedicated server, but compared with a VPS, where resources such as server capacity and processing performance are fixed by plan, a cloud server has the characteristic of high flexibility because resources are not fixed and can be increased quickly when access unexpectedly surges. Representative examples include AWS and GCP.

Server Uses

The four types of servers introduced above are servers as computers.
By contrast, servers as software provide various functions depending on their purpose.

Web Server

A web server returns data on the web to clients according to requests from clients. It includes HTML files that make up a website (homepage), CSS files that adjust the page structure and design, image files, and similar resources. When a client requests stored web page information, the server returns and sends the corresponding data.

Mail Server

A mail server is used when sending and receiving mail.
Servers are divided into sending and receiving roles. SMTP servers are used as sending servers, and IMAP or POP3 servers are used as receiving servers. The reason mail can be received without problems after turning the power back on, even if mail was sent while the power was off, is that the incoming mail data is stored inside the mail server.

Difference Between POP3 Servers and IMAP Servers

The major difference between POP3 servers and IMAP servers is whether mail data continues to be stored on the server. With a POP3 server, mail data is stored on the server at first, but when it is received by mail software, the data is downloaded to the PC or smartphone and then deleted from the server. The reason you can read mail received in the past even when not connected to the network is that the mail data is stored on the device. On the other hand, an IMAP server leaves data on the server. Therefore, when using multiple devices such as PCs and smartphones, the same mail can be read. In Gmail or Naver Mail, the reason you can read the same mail from any device after logging in is that an IMAP server is used.

DNS Server

A DNS (Domain Name System) server is used to connect IP addresses and domains.
An IP address is a unique number assigned to a device connected to a network and is like an address on the web. However, because a string of numbers is difficult for people to remember, a domain is the name attached to that string of numbers.

For domains and DNS servers, the explanatory manga distributed by JPRS (Japan Registry Services Co., Ltd.), a company that manages jp domains nationwide, is easy to understand.

File Server

A file server is used to share and use data on a network.
Through a file server, data sharing can be performed smoothly across multiple servers and various operating systems.

Database Server

A database server is a server equipped with a database and running a database management system. It receives requests from clients, searches organized information in the database, and sends processing results back.

FTP Server

An FTP (File Transfer Protocol) server is also called a “file transfer protocol” server and is used to send and receive files to and from a web server.
Unlike a web server that sends files only when there is a request, an FTP server allows uploads from a client to a server and downloads from a server. When creating a website, an FTP server is used to upload new files to a web server.

SSH Server

An SSH (Secure Shell) server is a server that encrypts client information.
In early remote communication methods, personal information was not encrypted, so information could sometimes leak. When sending personal information such as passwords or passcodes, SSH servers made it possible to encrypt the information first and then send it remotely and safely.