Are you trying to access the FMP server across the internet, or just within your local domain? If you're trying to access it across the internet, and you don't have a static IP on the web, you can check out www.no-ip.org - they provide a service that allows you to connect to a computer with a dynamic IP on the web. For a server, this is not the ideal solution. It would be much better if you established a static IP.
Also, its a good idea to avoid accessing the server via an IP address, which can change. Its a better practice to access the server using a name like fmp.mydomain.com. Then, you can change the ip number behind that name without having to change other things, since programs will still point to fmp.mydomain.com, not the underlying number.
Note: FMP is very, very slow across the internet. If you have to support users across the internet, check out Terminal Services and Citrix. These programs provide ultra lightweight communication protocols that allow users to login to FileMaker on your server from across the internet, but work much much faster than just FileMaker remotely.