Telnet (Telecommunication Network) is a network protocol used for character-based data exchange over a TCP connection.
By default port 23 is used.
Telnet is also be used to configure COM-Servers. Prerequisite is that a valid IP-Address has been assigned through which it can be addressed.
Deviating from the standard sometimes different port number are used ( Wiesemann und Theis uses port 1111 )

open-source Telnet Client