Unable to PING second NIC

I have installed a second NIC in my NT 4.0(sp6) server to accomodate our DSL connection. Server runs DHCP and assigns internal IP addresses in the 192.xxx.xxx.xxx range. DSL line has completely different IP numbering assigned, something like 12.xxx.xxx.xxx. I can PING both cards from the server, obviously, and I can access the internet through the DSL line at the server. I cannot, however, ping or connect to the internet from any desktop on the LAN. IP forwarding is enabled. What am I missing?