Using DHCP

I have a DSL router that I recently had setup with a DHCP scope to dynamically assign IPs to my mixed NT and Macintosh network. Previously IPs had been static on our private network. The problem: only two of the 20 macs configured the connection using DHCP correctly. The others pointed to the router as the name server, which has a private network address, instead of pointing to the ISP’s name server on the net, although they did correctly get an IP from the scope, the subnet mask is correct, asis the gateway (the router). I tried trashing the TCP/IP pref to no avail. The OS version doesn’t seem to make a difference. All the NTs are working correctly.