netstat

There's a few parameters to netstat that are useful for this :

• • -l or --listening shows only the sockets currently listening for incoming connection.

  • -a or --all shows all sockets currently in use.

  • -t or --tcp shows the tcp sockets.

  • -u or --udp shows the udp sockets.

  • -n or --numeric shows the hosts and ports as numbers, instead of resolving in dns and looking in /etc/services.

You use a mix of these to get what you want. To know which port numbers are currently in use, use one of these:

netstat -atn # For tcp netstat -aun # For udp netstat -atun # For both

In the output all port mentioned are in use either listening for incoming connection or connected to a peer** all others are closed. TCP and UDP ports are 16 bits wide (they go from 1-65535)