Definition

Telnet

Contributor(s): Laura Fitzgibbons and Alexis Layton

Telnet is a network protocol used to virtually access a computer and to provide a two-way, collaborative and text-based communication channel between two machines.

It follows a user command Transmission Control Protocol/Internet Protocol (TCP/IP) networking protocol for creating remote sessions. On the web, Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP) simply enable users to request specific files from remote computers, while, through Telnet, users can log on as a regular user with the privileges they are granted to the specific applications and data on that computer.

An abstract example of the syntax for a Telnet command request is as follows:

telnet the.libraryat.whatis.edu

The result of this request would be an invitation to log on with a user ID, and then the program would prompt the user for a password. If accepted, the user is granted access to the remote host.

Telnet is most likely to be used by program developers and anyone who has a need to use specific applications or data located at a remote machine.

How Telnet works

Telnet is a type of client-server protocol that can be used to open a command line on a remote computer, typically a server. Users can utilize this tool to ping a port and find out whether it is open. Telnet works with what is called a virtual terminal connection emulator, or an abstract instance of a connection to a computer, using standard protocols to act like a physical terminal connected to a machine. FTP may also be used along with Telnet for users working to send data files.

Users connect remotely to a machine using Telnet, sometimes referred to as Telnetting into the system. They are prompted to enter their username and password combination to access the remote computer, which enables the running of command lines as if logged in to the computer in person. Despite the physical location of users, their IP address will match the computer logged in to rather than the one physically used to connect.

Uses of Telnet

Telnet can be used for a variety of activities on a server, including editing files, running various programs and checking email.

Some servers enable remote connections using Telnet to access public data to play simple games or look up weather reports. Many of these features exist for nostalgic fun or because they still have compatibility with older systems that need access to specific data.

Users are also able to connect to any software that utilizes text-based, unencrypted protocols via Telnet, from web servers to ports. Users can open a command prompt on the remote machine, type the word telnet and the remote machine's name or IP address, and the telnet connection will ping the port to see if it is open or not. An open port will show a blank screen, while an error message that says the port is connecting means that it is closed.

Security

Telnet is not a secure protocol and is unencrypted. By monitoring a user's connection, anyone can access a person's username, password and other private information that is typed over the Telnet session in plaintext. With this information, access can be gained to the user's device.

SSH and related protocols

Some modern systems enable only command-line connections using Secure Shell (SSH), an encrypted tool similar to Telnet, or through a virtual private network (VPN). Because of security concerns, many professional organizations require use of SSH, PuTTy or other options instead of Telnet. SSH is the most commonly used alternative, largely because it encrypts all the traffic that passes over the communication channel.

Also, unlike newer protocols, Telnet does not support graphical user interfaces (GUIs), making it incompatible with many modern programs, from spreadsheets and web browsers to word processors and simulation software. Because those programs run complex graphical interfaces, large amounts of data, especially visual data, would be lost through a Telnet session connection.

History of Telnet

Telnet was originally run over Network Control Program (NCP) protocols. It was later called Teletype Over Network Protocol, or TONP. While it was used informally for some time, it was officially established on March 5, 1973, in published papers.

In early forms, Telnet used American Standard Code for Information Interchange (ASCII) delivered over an 8-bit channel to enable remote computers to communicate with basic text.

Over time, several Telnet extensions were created. Telnet has been around as a tool for programmers for several decades. The first version of Telnet was created for the Advanced Research Projects Agency Network (ARPANET), the precursor to the modern internet, in the 1960s. It was one of the first tools created to link computers remotely over large distances. A Telnet protocol was developed by researchers and professionals in 1971 followed by the Telnet system in 1983.

This was last updated in April 2020

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wow thanks Margaret! Really eye-opening analysis on telnet! Keep up the good work!
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