This is a quick guide to using the netcat command.
Netcat is a really handy tool to check whether a certain port is open on a box, here's an example on how to do this. For example here's how to check if port 8140 is open on the box, example.com:
$ nc -v example.com 8140 Ncat: Version 6.40 ( http://nmap.org/ncat ) Ncat: Connected to x.x.x.x:8140.
Here's another example, but this time to check whether a port is open on the localhost:
$ nc -v 127.0.0.1 22 Ncat: Version 6.40 ( http://nmap.org/ncat ) Ncat: Connected to 127.0.0.1:22. SSH-2.0-OpenSSH_6.6.1
Note: nc can't be used to check the connection of UDP ports, for example the ntp port is a udp port, with port number 123. If you try you'll get a 'no route to host' error message.