Why is my IP address invalid?

Question

I know there are several questions already posted regarding this problem, but not was resolved, so I decided to post mine in the hopes it would get an answer.
I've successfully created an in-home network using my Airport Extreme Base Station, but there's one problem and it's a big one. The base station refuses to recognize my modem's IP address as a valid one. All of my devices recognize the network, properly request access but then can't hit the Internet.
Right now, my Ethernet cable is directly linked to my Motorola Surfboard modem and Internet access is instantaneous. When I plug the Ethernet cable into my Airport, nothing.
Can anyone offer any suggestions as to what I've done wrong and how I can fix the problem? I've powered down both the modem and base station, restored the base station's settings to factory defaults and start over ... all to no avail.
Thanks in advance for your help.

Answer 1

I experienced the very same symptoms as Logan describe in the original problem.
Problem Statement:
Mac Pro (OS 10.6.2), connected directly to Motorola Surfboard cable modem (SB5120) via ethernet cable, has no problem acquiring DHCP IP address and internet access.
Add Apple Airport Extreme Base station, connect the Airport to the Motorola Surfboard modem (release the IP address), connect the Mac Pro to the Airport Express via ethernet cable; results in no connection (Invalid IP address reported by Airport Extreme Base Station). The Airport extreme, configured for a DHCP connection will not advertise a valid IP address. The Airport keeps providing a 169.* Private network IP address.
Solution:
Using Airport Utility configure the Airport Extreme Base station such that the "Ethernet WAN Port" is Not set to the default value of Automatic. Instead use the specific speed of, "100Mbps/Full Duplex".
Reason:
Despite the fact that the Motorola Surfboard modem advertises to the Airport Extreme that it can support "1000Mbps/Full Duplex" connectivity, my equipment cannot. By selecting the 'Automatic' setting on the Airport Extreme, I consistently will not receive a valid IP address. By changing the Ethernet WAN Port, and specifically dial-in a speed, everything works great. I have Comcast as a provider of high-speed internet.
This took me some time to troubleshoot, and the support of a resident Apple technical expert at work, who already have a valid network set-up and configured with an older version Airport. If this can help someone save half of the time that I wasted, this is a useful post.
Enjoy...