Comcast, Surfboard SB5101 and Airport Extreme (4th Gen) suddenly quit working

Your modem is not distributing IP addresses. The 169.254.51.249 address is "self-assigned" and worthless. As you indicated in step 7, nothing else is going to matter since your Extreme is not connected to the Internet.

Your modem configuration and status pages ought to look like this:

Regardless of what your cable company says if the modem is not distributing IP addresses it's not going to work. You might need to get them to replace it. Until then try powering it down for a good long time, at least several minutes, and let your Extreme rest also. Then, power up the modem, wait a few moments, followed by the Extreme and all other network devices in that order.

It sounds as though you changed the Extreme's settings so you may have to verify them again after doing this. Following that it may be required to perform another power down exercise.

Leave the Extreme connected to the modem during this power down. Connecting anything else to the modem will require that you power it down again so it "forgets" the device that was last using it.

If all else fails, perform a "hard reset" on the Extreme. It is possible an internal parameter has become corrupted, in which case the "hard reset" is the only way to clear the fault.

Make sure you do it properly so the "hard reset" is effective: http://support.apple.com/kb/HT3728

In addition to its own IP address the SB5101 distributes exactly one address to whatever is connected to it - generally, a router. Those IP addresses do not originate from beyond the modem.

Confusion understandably results when trying to diagnose problems like yours. If you bypass your Extreme by attaching a computer directly to the Surfboard so you can get online, then subsequently detach your computer and reattach the Extreme, it is certain not to work because the modem is not a router. It gave its one and only IP address to your computer and will continue to look for it at that address. Since it is no longer there, nothing happens. In the meantime your Extreme gets ignored, gives up, and assigns itself an IP address so nothing happens either. That's the reason for the power down.

I have heard anecdotes that some modems that take a long time to reset themselves; thirty minutes or more, but that seems unreasonably long.

Yes, you did everything I can think of and the only conclusion I can draw is that the Extreme's WAN port is NG. This would appear consistent with everything else you did.

It is strange that the failure would have coincided with the power going out.

The Apple Store ought to be able to confirm your diagnosis.

I ran into this very same issue just recently. My new Airport Express will only get a self-assigned IP address with the Surfboard modem. Funny enough, it works fine when I hooked it up to our U-Verse router so I know the unit is good.

Even more strange is that our older Airport Express base station works just fine with the Surfboard. Multiple base station resets did not work at all.

So now I'm using my old Airport Express and my new one is collecting dust until I can figure this out.

Turns out my Airport Express was defective. Made an appointment with a Genius at the Apple Store and a fault immediately showed up on the diagnostic so Apple replaced it there on the spot. I was very happy.

Your problem sounds a lot like mine. Today I upgraded to faster internet and my 2 ipads, 2 laptops, and iMac were all working well prior to the exchange of cable modem to an Arris TM722 with a comcast reference on the side. Now I can only connect the laptops individually the iPads are not connecting at all. My wife just noticed that the phone was not working so I am hoping it is the cable modem my Airport is only a few months old. Frustrating.

Whenever you change the modem on a network, the AirPort router must be reset back to default settings and then set up again, so that it can associate with the new modem correctly.

Did you do this?

Thanks Bob I did eventually reset to factory and everything started working as it should. I passed the info along to my internet service providers so they know what to do for the next Apple system user.