Awhile back I picked up an Airport Extreme Base Station to replace a Linksys Wireless Access Point. Unfortunately I was brain-dead while getting everything setup and couldn't get it to talk to the Comcast Cable Modem so I ended up putting it behind the Linksys WAP and turning off the Wireless on the Linksys device.
Tonight I fixed the issue. Turns out I needed to turn off the Cable Modem (as well as remove the built-in battery backup) for a few minutes while keeping the AEBS turned off. This allowed for the Comcast Modem to clear both the MAC address of the Linksys and the IP addresses. Next I turned on the Cable modem, waited till it had a good connection then turned on the AEBS. The AEBS was then able to pick up a valid IP address and everything was good to go.
Thank you thank you thank
Thank you thank you thank you! How many hours have I spent trying to make this work. Changing every setting on my computer and base station. Plugging unplugging and resetting over and over. But taking the battery out really did it! Bless you.