If you are running Win2000 AD with DNS on the server, the server has an IP address which could be your public IP , but probably DHCP from your cable modem. The server NIC has as its default gateway either the cable modem IP (if it has one) or the router IP address of your ISP's router. There should be NO DNS ADDRESS ENTERED IN THE SERVER NIC. (The server is its own DNS source.)
The DNS part of the AD server has a forwarder set to the ISP DNS server address.
So the DNS request goes from the AD DNS server, if it cannot find the address, to the ISP DNS. If you have a LAN, then you have clients, unless its a test network, you should run DHCP on the AD server and pass the IP address, the default gateway address, and your DNS server (AD server) IP address to them using this.
Alex