Telnet is the protocol used to log into and operate remote systems over a network without built-in encryption. It presents a remote terminal, allowing a user to enter commands on another computer as though working at its local command line.
Telnet was developed for ARPANET-era networking and was formally standardized through a series of Internet standards in the 1970s and 1980s. It generally operates over TCP, traditionally using port 23. The protocol can support bidirectional communication and negotiates terminal features between the client and server.
The major catch is security: traditional Telnet sends usernames, passwords, and session data in plaintext. Attackers who can observe the connection may capture credentials or commands. SSH replaced Telnet for most secure remote administration because it provides encryption and stronger authentication. Telnet still appears in legacy equipment, specialized systems, and network troubleshooting, but an open Telnet login on the public Internet is considered unsafe.