No change should be made to any IP address settings in the host file or the application (filemaker pro or server) hosting it. You should specify the IP Address of the internet router that your IT guy is setting up with port forwarding on your client machine when using the internet to connect to it.
There are settings on the hosted file and the application (if hosted by fileMaker Pro or FileMaker Pro Advanced) that do need to be set before the file can be accessed from other machines:
Open Sharing from the File menu and select FileMaker Network.
If this is the application that will host the file, turn sharing on.
Now select the file to be hosted and specify "all users".