A dynamic IP address is handed out to your Cable Modem|Router or DSL Modem|Router by your ISP.
Generally, dynamic IP addresses seldom change, if the power goes out in your neighborhood or city, it may change.
If your router fails and is replaced, your IP address may change.
A static IP address has an advantage in that it will never change unless your ISP goes out of business or you change provider.
http://whatismyipaddress.com gives your Modem|Router IP address.
The internet can talk to your IP address (Router|Modem), your router has to be told which computer to send requests for Filemaker to.
Unless, you have only one computer plugged directly into the Modem|Router. (mostly)
http://portforward.com explains how for many routers
This question has been answered many times here.
This Google lists the threads on this site.