Question : Problem: Airport Extreme: No IP address from Comcast cable modem

I installed an Airport Extreme yesterday, and keep getting an error message that it is not getting an IP address via DHCP.

Previously the cable modem was connected directly to an iMac, which was had Internet Sharing selected in the Sharing pref pane. After the Airport Extreme installation failure I reverted to the original configuration, and everything worked as before.

When trying to configure the Airport Extreme, I connected the Ethernet cable from the modem to the WAN port of the Airport Extreme, and connected the Airport Extreme to the iMac via Ethernet. I was able to access the Internet on the iMac, but not with an iBook connected to the newly configured wireless network. The iBook could see and access the network, but could not access the Internet. I need help!

Answer : Problem: Airport Extreme: No IP address from Comcast cable modem

Some Comcast cable modem / routers store the MAC address of the last device connected.  Try hard resetting the Comcast cable modem/router (procedure varies by type but info available on Comcast site).  I've also ran into this issue with a FIOS router 2 years ago.