History of Telnet

History and standards

Telnet is a client-server protocol, based on a 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 (telnet) is listening. Telnet, however, predates TCP/IP and was originally run over Network Control Program (NCP) protocols.
Before March 5, 1973, Telnet was an ad-hoc protocol with no official definition.^[1] 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^[2] with the publication of two NIC documents: Telnet Protocol Specification, NIC #15372, and Telnet Option Specifications, NIC #15373.
Because of negotiable options protocol architecture, many extensions were made for it, some of which have been adopted as Internet standards, IETF documents STD 27 through STD 32. Some extensions have been widely implemented and others are proposed standards on the IETF standards track (see below)