If you want to see which process is using which port (in this case, port 53), you can do it with the following command:
# netstat -tulpn| grep :53
This gives the following output:
tcp 0 0 :::53 :::* LISTEN 1234/named
As you can see, dns (named) is using port 53 on PID 1234.
You can also do it with other ports by replacing the number (53) by another port.
