Connecting ISDN Router to network

I have a ISDN Router that is connected to our network, it is also the DHCP for the network. Can I go from the router to a NIC in the server and allow the server to provied the DHCP service and another NIC to the network via a switch? If so what settings should the sever have running other then DHCP? I think right now if I turned the server off everyone still has access to the Internet.