Test if a port on a remote system is reachable

With telnet:

telnet webserver.example.com 80

With bash:

cat < /dev/tcp/webserver.example.com/80

Replace tcp with udp, depending on what you want.

With netcat:

nc webserver.example.com 80 &> /dev/null; echo $?

If the port is open, you will get an output of 0. If it’s closed, it’s a 1.