are you connected directly to the cabl modem?
on the cable modem that I have, (telewest in the UK) when you first setup the connection that MAC addresss of the connecting machine is registered, this makes it difficult to change hardware in future.
now that I have a router attached to the that actual cable modem I have to use mac spoofing to make the router tell the modem that it has the same MAC address as the computer did when the computer was plugged straight into the modem...
limited or no connectivity implies that there is no IP address recieved,
if you don't have a DHCP serer try setting the IP address manually.
you may want to look at the instruction manual for the router to get an idea of what the settings should be though.