Custom resolve an URL?

A server is accessed externally using a xxxx.dyndns.org address and is connected to the internet via a DSL router with a dynamic IP. Clients that need to have access to the server both internally and externally need to for example configure two mail accounts, one with the server set to xxxxx.dyndns.org and the other one set to 192.168.0.x. I need advice or some starting points on how to set up DNS (if this is the service that I need) on Mac OS X Server 10.3.9 so that it does the same job as the internet provider's DNS server but resolves the xxxxx.dyndns.org to 192.168.0.x for local clients. Also, why is it that on networks with a DrayTek router the computer name in the terminal prompt is set to the last segment of the IP address? Thanks in advance.