What is Telnet?
Telnet is a network protocol used to virtually access a computer and provide a two-way, collaborative and text-based communication channel between two machines.
It follows a user command TCP/IP networking protocol that creates remote sessions. On the web, HTTP and File Transfer Protocol (FTP) enable users to request specific files from remote computers. With Telnet, users can log on as a regular user with privileges that allow them to access the specific applications and data on that computer.
An abstract example of the syntax for a Telnet command request is as follows:
The result of this request would be an invitation to log on with a user ID, and the program would prompt the user for a password. If the computer accepts the user, it grants them access to the remote host.
Program developers and anyone who needs to use specific applications or data located at a remote machines are more likely to use Telnet.
How Telnet works
Telnet is a type of client-server protocol that opens a command line on a remote computer, typically a server. Users can use this tool to ping a port and find out if it's open.
Telnet works with a virtual terminal connection emulator, which is an abstract instance of a connection to a computer, and uses standard protocols to act like a physical terminal connected to a machine. Telnet also sometimes uses FTP to help users send data files.
Users connect remotely to a machine with Telnet. This is sometimes referred to as Telnetting into the system. Telnet prompts users to enter their usernames and passwords to access the remote computer, which enables command lines to run as if users are logged in to the computers in person. A user's IP address matches the computer they're logged in to rather than the one physically used to connect.
Uses of Telnet
Users employ Telnet for a variety of activities on a server, such as to edit files, run various programs and check email.
Some servers use Telnet to enable remote connections to access public data and play games or look up weather reports. These features exist for nostalgic fun or because they have compatibility with older systems that need access to specific data.
Users are also able to connect to any software that uses text-based, unencrypted protocols via Telnet, from web servers to ports. Users can open a command prompt on the remote machine, type the command telnet and the remote machine's name or IP address, and the Telnet connection pings the port to see if it's open. An open port shows a blank screen, while an error message that says the port is connecting means the port is closed.
Telnet is neither encrypted nor secure. Because Telnet monitors a user's connection, anyone can access a person's username, password and other private information typed over the Telnet session in plaintext. A bad actor can use this information to gain access to a 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. Because of security concerns, many professional organizations require the use of secure options, like SSH or PuTTy, 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 doesn't support graphical user interfaces. This means Telnet isn't compatible with many modern programs, such as spreadsheets, web browsers and word processors. These programs run complex graphical interfaces, so Telnet session connections typically lose large amounts of data -- especially visual data.
History of Telnet
Telnet originally ran over Network Control Program protocols. It was later called Teletype Over Network Protocol. Network professionals informally used Telnet for some time before it was officially established on March 5, 1973, in published papers.
In early forms, Telnet used American Standard Code for Information Interchange 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, the precursor to the modern internet, in the 1960s. It was one of the first tools created to link computers remotely over large distances. Researchers and professionals developed a Telnet protocol in 1971 followed by the Telnet system in 1983.
Editor's note: This article was reformatted to improve the reader experience.