Telnet is a communications protocol that is compatible with TCP/IP (the Internet), and can be used over LAN's and other computer networks. The primary purpose of Telnet was/is to provide an interface for terminal devices and processes, which can provide a remote login to a computer via the Internet. Telnet was first discussed in RFC15 (1969) and that document states that it's purpose was to provide "user program access, a convenient means for direct network access from the terminal". Although the Telnet term is often referred to as meaning "Terminal Network" it is infact believed to refer to meaning "Teletype Network". Telnet is defined as bi-directional and uses a eight-bit byte data transmission connection that is typically provided via a command-line interface (shown below).
Telnet is one of the oldest protocols of the Internet Protocol Suite - it was probably the first application demonstrated on the four-IMP ARPANET - and was designed throughout the 1970's until a final specification was released in 1983; which outlined Network Virtual Terminal, negotiation and commands. Telnet was developed before the creation of TCP/IP and the Internet, and it was designed as a service for the APRANET computer network that initially used the Network Control Program (NCP). In 1983, the Network Control Program (NCP) was replaced by the TCP/IP protocol suite, and Telnet currently uses the TCP transport protocol (on port 23) to establish a connection across IP networks.
Telnet is a part of the application layer of the Internet Protocol Suite (TCP/IP) and relys upon a connection-based data transport (TCP). Like most application layer protocols, Telnet relays upon a client-server model: where terminals use a Telnet program (client) to remotely access/login to a server (that uses a telnet server program). Telnet programs use a facility named a Network Virtual Terminal (NVT) to establish communication across TCP/IP networks. NVT uses seven-bit ASCII character data / codes that are sent upon an eight-bit channel. Various commands can be inputted into a Telnet client program to communicate between the client and server.
The drawback to Telnet is it's age: usability and security. Telnet does not
feature advanced functions like encryption: this means that passwords are
not encrypted and can be intercepted and read when Telnet establishes a connection
to a device. This is one of the main reasons why Telnet use has dropped sharply.