unless you are running a firewall on the mac, then all ports that are listening are open.
if you run FM server, its ports are open to your LAN.
what you are probably hoping to do is open ports to the internet.
to do that, you need to map ports in your router to your server.
Make sure you mac pro is set on a manual LAN address (so it doesnt change).
Here's a hypothetical:
your public IP is: 188.8.131.52 (get this with whatismyip.com)
your mac pro is: 10.1.10.2
you need to access filemaker tcp port 5003 from the public internet
in your router, you map public port 5003 to private 5003 at 10.1.10.2
From the internet, your fm client would access: 184.108.40.206
hope that gets you started