DOES YOUR IP ADDRESS CHANGE FREQUENTLY? If so, you may well benefit from using a service like Dyn.com. Unfortunately, my experience with Dynamic IP addressing has been negative so far. I am using it for an NAS System running a version of linux, however, so maybe that is part of the problem. Or maybe it's just the router–more troubleshooting will tell.
Douglas Alders of HomeBase, however, has had good luck using a dynamic IP service with his clients:
Once you have your FileMaker Server setup and accessible remotely, I recommend setting up Dyn.com Remote Access service. Their tagline is “Access your device or files using a unique hostname, instead of a confusing (and ever-changing) IP address.”
The reason you may need a service like this is because in many cases your Internet provider doesn’t want you to host services on your home or small business line. They will frequently change your network IP address to prevent you from hosting services.
What that means to your users, is that periodically they will have to call you up and complain ‘They changed the IP address again. What is it now?”, instead of being productive. Your Internet service provider (ISP) would rather upgrade you to a business package with a fixed IP address at an additional cost.
Follow the link for step by step instructions on how to set it up properly and you'll no doubt have good, reliable results.
More... How to Maintain a Connection to FileMaker Server on a Dynamic IP Address with DynDNS Updater | HomeBase Software.