Router not giving internet

Hi, guys. I'm currently at a friends house and I brought my wireless Linksys WRT54G2. I have the router connected to his Motorola SBV5121 Cable Modem. When I have it connected to the modem, and something else connected to the router through another ethernet cable, that thing won't receive internet connection. I can plug the computer directly into the modem and get internet but when I use the router as a HUB for ethernet cables, the things I connect to it don't get any internet connection. The router is set on DHCP Automatic internet connection. Please help, it's much appreciated.

Comcast, my cable provider, only allows one MAC address to send to the modem. When I first got them I anticipated that problem and (since I had to use a computer to activate my modem) I changed the MAC address of my computer's ethernet card to that of my router. Activated, plugged the router in, worked. Now if I plug anything into the modem, without changing its MAC address, it doesn't work. When I got a new modem I had to do the same thing on my computer again.

So find out the address on the computer (various ways, try ipconfig) and configure your router to use that temporarily.
Or you can call up the cable company and talk to them - they can probably whitelist the router.

Also, if it's a MAC address thing many routers has a "mac address clone" feature that will copy your computer MAC address into the router's MAC address so cable internet providers won't complain. I think that's available in your model linksys, it's available in my linksys wrt54gs.

Check the status in the router to see if you get an external IP address too. Assuming you can talk to your router, which you should be able to do regardless of the WAN side.

This is a video showing how to setup a wrt54g2 router. It may help you>>>
http://www.youtube.com/watch?v=1MBfKJArOCw

Try unplugging the modem, leaving it off for 5 min. Then plug in the modem let it startup completely and then connect the router. Modems normally cache the Mac address of the first device that is plugged into it. If this is the case Rebooting it flushes this cached value.