devkuma – Internet

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.

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

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.

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