Ask Max: My Comcast modem won't provide a working IP address to my AirPort Base Station...
I just tried to replace and older (dome shaped) Airport wireless system in the house. The older system was working... Don't ask why I decided to switch to a newer model... Anyway, I have spent much of the weekend on the phone with both AppleCare and Comcast to no avail. It appears the Comcast Modem will not give a working IP address to the airport.
I have researched a number of trouble shooting sites and it appears this is not a unique problem. However, I have gone through MOST if not all of the steps that the sites recommend to fix the issue.
All computers are running the latest OS from MAC. It is an entire MAC system including Apple TV.
I am fairly sure it is not an issue with your Apple equipment. This is because I have exactly the same setup in my home and it works reliably. The one exception is that I do not like the equipment Comcast provides. I instead use my own Cable modem. This is the Linksys router I have used for years and like its rock -solid performance.
Before you run our and get a new modem, try these two possible fixes. First off power everything. Let the cable modem sit, without power, for five minutes and then power it on again. After it is fully on and connected to the Comcast network, power on the Airport Base station. Make sure your cable modem is the only Ethernet device connected to the Airport base station and that you have it connected to the WAN port. That is the port off to one side with a dotted circle above it. Now you can connect your devices.
If that fails you may have been given a Cable modem that has its own DHCP / NAT serve and it is in conflict with your Airport's DHCP / NAT service. Here is the process for disabling DHCP and NAT on the Airport Base Station.
If all my suggestions fail, get the Linksys modem. You may even save money on your monthly bill.