Are you using g5's or intels?.
Watch out if you are using the 'other' network port on the intel - especially if it had a non-artnet ip address. if not in use - turn it off. I have issues with multiple network cards and artnet/catalyst constantly...

IP address range = 2.0.0.x or 10.0.0.x. You mention that you are using a 'router' - is is actually a router or a switch or hub? if your DMX to artnet hardware is connected to 1 side of a rouer, it is possible that the network ip range is being blocked 2.0.0.x is routable 10.0.0.x is NOT.

What are your network setings, dmx patch...

Simon