TCP 23: Telnet

Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection.

-- Wikipedia

Basics

telnet <ip> <port>

Brute force

hydra -l root -P /usr/share/seclists/Passwords/Common-Credentials/10-million-password-list-top-100.txt <ip> telnet