IIS or DNS Problem

We are hosting our own website, using an IP provided by our DSL provider. If a user types in the web address for our site, it will switch to the IP address of the host machine. Inside our LAN, users must type in the IP to view the website. (I hope that made sense) The system hosting the web site is also connected to the LAN, so it can access an internal database for our sales agents use. The machine in question is a W2k Pro. Our LAN is NT based, and we run our own DNS. Any help would be appreciated.