Telnet

Telnet is a basic Internet protocol that simulates the functionality of a character-by-character terminal, running over the Transmission Control Protocol. Even in a Web era, it remains important for accessing maintenance interfaces on network infrastructure.

Reverse telnet, in which a virtual window can speak to a number of serial interfaces through an appropriate application gateway, is a common means of accessing the consoles of multiple colocated routers or other network elements.

The well-known TCP port for Telnet is numbered 23. Most Telnet clients can go to alternate ports, and a Telnet server can redirect the session to a different address and port.

Since Internet application protocols are usually character-based, it can be used to test a variety of application protocols; "telnetting to Port 25", for example, allows Simple Mail Transfer Protocol messages to be constructed at a window and sent to the SMTP server.