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What is Telnet?

Telnet, developed in 1969, is a protocol that provides a command line interface for communication with a remote device or server, sometimes employed for remote management but also for initial device setup like network hardware. Telnet stands for Teletype Network, but it can also be used as a verb; 'to telnet' is to establish a connection using the Telnet protocol. Telnet can be used to test or troubleshoot remote web or mail servers, as well as for remote access to MUDs (multi-user dungeon games) and trusted internal networks.[1]

While Telnet pales in comparison to modern networking technology, it was revolutionary in 1969, and Telnet helped pave the way for the World Wide Web in 1989. In time, insecure Telnet evolved into the newer SSH network protocol, which modern network administrators use to manage Linux and Unix computers from a distance. SSH provides strong authentication and secures encrypted data communications between computers over an insecure network.

Unlike Firefox or Google Chrome screens, Telnet screens are unremarkable to view. Telnet is all about typing on a keyboard. It has none of the graphic elements we expect from web pages today. Telnet commands can be cryptic, with example commands including z and prompt% fg. Most modern users would find Telnet screens to be archaic and slow.

Telnet is rarely used to connect computers anymore because of its lack of security. However, it is still functional; there's a Telnet client in Windows (10, 8, 7, and Vista), although you may have to enable Telnet first.

History and Standards of Telnet[2]

Telnet is a client-server protocol, based on reliable connection-oriented transport. Typically, this protocol is used to establish a connection to Transmission Control Protocol (TCP) port number 23, where a Telnet server application (telnetd) is listening. Telnet, however, predates TCP/IP and was originally run over Network Control Program (NCP) protocols.

Even though Telnet was an ad hoc protocol with no official definition until March 5, 1973, the name actually referred to Teletype Over Network Protocol as the RFC 206 (NIC 7176) on Telnet makes the connection clear:

The TELNET protocol is based upon the notion of a virtual teletype, employing a 7-bit ASCII character set. The primary function of a User TELNET, then, is to provide the means by which its users can 'hit' all the keys on that virtual teletype.

Essentially, it used an 8-bit channel to exchange 7-bit ASCII data. Any byte with the high-bit set was a special Telnet character. On March 5, 1973, a Telnet protocol standard was defined at UCLA with the publication of two NIC documents: Telnet Protocol Specification, NIC 15372, and Telnet Option Specifications, NIC 15373.

Many extensions were made for Telnet because of its negotiable options protocol architecture. Some of these extensions have been adopted as Internet standards and IETF documents STD 27 through STD 32. Some extensions have been widely implemented and others are proposed standards on the IETF standards track. Telnet is best understood in the context of a user with a simple terminal using the local Telnet program (known as the client program) to run a login session on a remote computer where the user's communications needs are handled by a Telnet server program.

How Telnet Works[3]

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.

See Also

TCP/IP (Transmission Control Protocol/Internet Protocol)
Network Protocol
Protocol Stack
Internet Protocol (IP)
File Transfer Protocol (FTP)
POP (Post Office Protocol)
VoIP (Voice Over Internet Protocol)
Extensible Authentication Protocol (EAP)
Hypertext Transfer Protocol (HTTP)
User Datagram Protocol (UDP)
Protocol Data Unit (PDU)
Network Control Program (NCP)
Transmission Control Protocol (TCP)
Time Triggered Protocol (TTP)
Business Transaction Protocol (BTP)
Common Alerting Protocol (CAP)
SMTP (Simple Mail Transfer Protocol)
Advanced Message Queuing Protocol (AMQP)
Simple Object Access Protocol (SOAP)
Internet Message Access Protocol (IMAP)
IP Network
Ethernet Industrial Protocol (Ethernet/IP)
Simple Network Management Protocol (SNMP)
Access Media Gateways (AMG)
Secure Sockets Layer (SSL)
Internet Message Access Protocol (IMAP)
Synchronous Optical Network (SONET)