Apple Support with a question and answer for the ages:
We have just set up a firewall on our network. Now I can’t access our Filemaker Pro Server over TCP/IP. In order to gain access to the server over TCP/IP, our network administrator needs to know what “port” the Filemaker Pro Server uses. Can you tell me what TCP/IP port number is used by the FileMaker Pro Server?
The Filemaker Pro Server, uses the TCP/IP port number of 5003.