No internet access for non domain user?

Currently i setup our Window server 2003 to connect our modem router and right under my 2k3, i have a switch an a router.

2k3 server is running NAT and my router under my server is doing the DHCP. So is there a way that i can setup to allow internet access to user that logon to our domain. Those that didn’t logon to our domain server will not have any internet access?