IMac AirPort assigns it's own IP

Question

I recently learned I could use my iMac's built-in AirPort card as a wifi hub for my iPod Touch. I tried it and it worked fine. I was able to browse the Internet from my iPod thru the iMac, which is connected to a DSL modem and AT&T broadband. "This will be handy when I get my iPad next month," I thought, and turned AirPort off until I needed it again.
A few weeks later I receive my iPad, turn AirPort back on, and enable Internet sharing. But something changed--now I get a message saying that AirPort is using a self-assigned IP address and cannot access the Internet. All the options on the AirPort settings page are grayed out. What happened and how do I fix it?

Answer 1

Well, my AirPort seems to be working now. Thanks for all the help from readers on this forum....not. What finally worked was an article at http://www.tuaw.com/2007/03/26/how-to-using-your-mac-as-a-nat-router/ I manually entered the IP address he used (under the "Adding another Ethernet port" section, strangely), and voila, my Touch and iPad can now access the web thru my Mac. I have no idea why it worked, or if there will be any unpleasant side effects.
I don't consider this question "answered" because I don't know why it worked fine one day, and then two weeks later required hours of research and a lot of experimentation to get it running. For all I know, turning the Mac off tonight will put me back at square 1 tomorrow. Baby steps.