DNS security and Cable modem

I would like some input on different ways to secure a 2000 server connected to a cable modem network.
Setup: single 2000 server running active directory and DNS server on a network of 6 XP pro stations. The server is the root DNA server for the forest. A Netgear router seperates a cablem modem from the network.
I would like pointers to setting up the server and workstations to allow for quick DNS resolution of local shares and internet addresses, and yet secure the DNS server from out side access.