What is Telnet used for?

In this tutorial, we are going to see What is Telnet used for? The Telnet protocol is a standard Internet protocol for connecting terminals and applications over the Internet. This protocol provides the basic rules for connecting a client (display and keyboard system) to a command interpreter (server side).

The Telnet protocol relies on a TCP connection to send data in ASCII format encoded on 8 bits with Telnet control sequences in between. It thus provides a communication-oriented, bi-directional (half-duplex), 8-bit coded system that is easy to implement.
 

 
 
The Telnet protocol is based on three fundamental concepts:

• The Network Virtual Terminal (NVT) paradigm;
• The principle of negotiated options;
• Negotiation rules.

This protocol is a basic protocol, on which some other protocols of the TCP/IP suite (FTP, SMTP, POP3, …) are based. The Telnet specifications do not mention authentication because Telnet is totally separate from the applications that use it (the FTP protocol defines an authentication sequence above Telnet). Also, the Telnet protocol is not a secure data transfer protocol, i.e. the data it carries travels in the clear over the network (unencrypted). When the Telnet protocol is used to connect a remote host to the machine on which it is implemented as a server, this protocol is assigned to port 23.

Besides the options and associated negotiation rules, the Telnet protocol specifications are basic. The transmission of data through Telnet only consists in transmitting the bytes in the TCP stream. When a byte 255 is transmitted, the next byte must be interpreted as a command. Byte 255 is thus called IAC (Interpret As Command).
 
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