TELNET (TELecommunication NETwork) is a network protocol used on the Internet.

TELNET is a client-server protocol, based on a reliable connection-oriented transport. This command will help us to find if a particular port on the server is open or not.

Example:

In the below example i am checking whether port # 22 is open or not in my system

[kamaraj@Kamaraj ~]$ telnet 127.0.0.1 22
Trying 127.0.0.1…
Connected to 127.0.0.1.
Escape character is '^]'.
SSH-2.0-OpenSSH_5.5
 

If you get the above message "Connected to 127.0.0.1" then it is open. If you get a message like "connection refused" then it the port is not open.