Q: I'm trying to find out which network ports I must open on my intranet firewalls to make one of our inhouse Windows client/server applications communicate properly. How can I list the remote network ports that a client-side application uses?

A: I recommend you use the Netstat command-line tool. If you run Netstat with the -an switch, the tool will display all existing connections on your local system and the local IP address, port information, remote IP address, and port information for each connection. The -a switch tells Netstat to display all connections and ports the system is listening to and the -n switch tells Netstat to display all network information in a numerical form, using the IP address and port number.

If you run Netstat with the -anob switch, it will also display the name of the local executable that is establishing the connection (the -b switch), and the local process ID of the process that owns the connection (the -o switch). If you replace the -n switch with the -f switch and run the command

netstat -afob

Netstat will display the Fully Qualified Domain Names (FQDNs) of the network entities involved instead of their IP addresses.

Netstat is available on all Windows systems by default. If you want a tool that shows you the network ports and associated processes and applications in a nice GUI, you can download Process Explorer or TCPView from Microsoft.