What the protocol is, its function and history

Telnet (from TELecommunication NETwork) is a TCP/IP stack network protocol that allows remote text-mode communication between computers. Its main function is to provide an interactive session where a user can access and control another device as if they were on a local terminal.

Created in 1969, Telnet was one of the first protocols developed for ARPANET (the network that gave rise to the Internet) and became fundamental for system and device administration in the 1970s, 1980s, and 1990s. Over time, it fell out of use due to lack of security, being replaced by more modern alternatives such as SSH (Secure Shell).

Implementation

Default port: 23/TCP.
Works at the application layer of the OSI model.
Format: Based on ASCII character exchange, without encryption.
RFC: Defined by RFC 854 (1983).
Architecture: Follows the client-server model

How it works

In practice, Telnet works relatively simply. The process starts when the client establishes a TCP connection to the server via port 23. Then a remote terminal session is initiated and the user must provide credentials such as username and password. After authentication, commands typed on the client are transmitted in plain text to the server, which processes them and returns the corresponding output. The session remains active as long as the user wants, normally ending with commands like exit or logout

Use cases

For many years, Telnet was widely used for remote access to Unix, Linux, and Windows servers, especially older versions of these systems. It also became common in network device administration, such as routers and switches, until SSH became the standard. Additionally, mainframes and some legacy devices still use Telnet today. Another practical application is in educational environments and network diagnostics, where it is used to test open ports and check service connectivity, such as running “telnet server.com 80” to see if a web server port is operational.

Encryption: the inherent problem

Telnet has no native encryption, which makes it extremely vulnerable. To solve this problem, more secure alternatives were developed. The main one is SSH (Secure Shell), created in the 1990s as a direct Telnet replacement. SSH offers the same functionality while ensuring data protection through strong authentication and full traffic encryption. Another, less common approach is using SSL/TLS to tunnel Telnet sessions, but in practice this is rarely used.

Advantages and disadvantages

Telnet’s advantages include simplicity, low resource usage, and compatibility with various older systems, which facilitated its adoption over the years. However, these benefits are outweighed by its disadvantages. The main one is the lack of encryption, exposing all transmitted data—including passwords—in plain text. This makes it vulnerable to attacks such as sniffing, which captures network packets, and hijacking, which takes over active sessions. For this reason, Telnet is considered obsolete and unsafe for use on open networks like the Internet.

Relation to other protocols

Telnet is part of the TCP/IP protocol family and uses TCP to ensure reliable communication. Like other protocols in this stack, such as HTTP, FTP, and SMTP, it relies on stable connections to perform its functions, but its distinguishing feature has always been terminal-mode interactivity. Due to security flaws, it was replaced by its natural successor, SSH, which retained Telnet’s conceptual base but added robust protection layers.

Functional example

Below is a Telnet connection simulator written in JavaScript.