Basics of networking

Telnet is the program used to connect to a remote server. Telnet is installed on server-class computers, but is also widely used on all kinds of network devices (e.g. switches, AccessPoint).

You can use telnet to check whether a particular service, e.g. SMTP, HTTP, is running on the remote computer.

To check the connectivity between the client computer and the server on the command line (terminal), issue the command:

telnet [address of server under test] [port of service specified]. 

If you want to check if there is a working STMP server on the computer www.nasa.gov and you will be able to connect to it from your computer and send mail, issue the command:

telnet www.nasa.gov 25

where 25 is the number of the TCP port on which the SMTP service (sending mail) listens.