Open command prompt/power-shell and type telnet ipaddress portnumber and hit enter.
If it shows you a blank screen, it means the the port you tried connecting to is open otherwise you get an error message stating “Could not open connection to host ” .
Example : telnet 192.168.100.1 81
If you get a message telnet is an unrecognized command , you need to install the feature.
On windows7/8, install it from programs and features, windows features – telnet client.
On windows servers install telnet client from server features.