Why did my Airport stop working?

December 29, 2010 9:42 PM

Quite suddenly, my modem works if I plug it into a machine, but not when I plug it into an Airport Express. Comcast says it's not their fault. Any idea what's going on? Every detail I can think of is inside.

Last night, when my wife and I got back from a trip, our internet was crawling along at an ungodly pace. I unplugged and replugged both my modem and my Airport Express to no avail.

This morning, I called Comcast to see if there was a problem with our service. They reset the modem on their side. Speed improved from .01 mbps to 6 mbps, but before our vacation we'd been getting twice that. The Comcast person on the phone had me plug my modem into my laptop, bypassing the Airport Express. This put the speed back to normal. His advice was to replace my old Airport Express.

I got a new Airport Express, came home and plugged the modem into it, but while the Airport shows a signal, I can't get online. When I tried to troubleshoot through the Airport Utility, it says the Airport is having an internet connection problem as it does not have a valid IP address.

Basically, both my wife's PC laptop and MacBook can see the wireless network the Airport created, but there's no internet. It's a similar issue to what davidmac had.

The internet still works when plugged directly into either computer. I tried the old Airport Express (which was functional but unusably slow before), but it doesn't work now either.

I've unplugged and replugged both my modem and the Airport. I also did a hard reset and tried the setup again. I called Comcast again, and they said they didn't see an error on their side so I had to take it up with Apple. Apple's feeling is that Comcast's issue for not passing along a valid IP address.

We've been getting wireless internet using an Airport with Comcast for the last five years so I'm unsure as to why things would suddenly stop working now.

Is there an obvious thing wrong that I'm missing? Are both the Airports suddenly duds? What do I do next?

posted by jaybeans to technology (12 answers total)

Unplug both the modem and the airport. Wait at least 5 minutes. Plug in the modem. Wait for it to come online. Plug in the AirPort. Try it then.

I'm not sure if this is an old wives' tale (or the digital equivalent thereof) or not, but it's worked for me. I'm told that the modem binds to the MAC address of whatever device is plugged in first, and won't connect to anything else until it's been powered down completely for a few minutes. This is one area of tech I don't know, so if it works, I'd be glad to hear the "why!?" rationale from someone who knows.
posted by Alterscape at 9:46 PM on December 29, 2010

Sadly, I gave this a try but with no luck. I really so wanted that to work.
posted by jaybeans at 9:59 PM on December 29, 2010

Comcast has been upgrading their local networks to handle DOCSIS 3.0. If your AirPort Express doesn't understand DOCSIS 3.O negotiations, or at least the higher bandwidth Ethernet connections that result from higher upstream bandwidth connections down network from the cable modem, perhaps it isn't able to reliably negotiate IP addresses, and network topology, including DNS resolvers, and gateway IP's, from your cable modem.
posted by paulsc at 10:12 PM on December 29, 2010 [1 favorite]

In my experience with Comcast when they first come out and set up service they configure the modem to work with a device with the same MAC address as your computer. Caveat, I'm completely unfamiliar with Airport Expresses, but the old one was probably set to the same MAC address as your computer, the new one will need to have something in it's settings changed so that it has the same MAC address as the old one did. I don't know how to do that though, but hopefully this sends you down the right rabbit hole.
posted by nulledge at 10:13 PM on December 29, 2010

When did you start your trip? There was a firmware update (7.5.2) for all the Airport devices back around the 14th seems to be causing various problems for some people - though it's working fine here for me.

If you want to try reverting (I'd suggest back to 7.4.2, for testing at least):

1. Fire up Airport Utility & connect to the Airport Express
2. Choose "Manual Setup"
3. In the "Summary" tab, click on the word "Version"
4. Choose 7.4.2. from the dropdown list
5. Follow the instructions to from there on in

If you don't see 7.4.2 in step 3, download 7.4.2 from here, choose "other" in the Airport Utility dropdown, and point to the downloaded firmware. I'm not aware of any major issues with 7.4.2 - 7.5.1 was for Airport Extremes & Time Capsules, while 7.5.2 fixes some minor Airplay, USB device, & 5GHz spec compliance issues.

Anyway, give that a shot and see if that improves things.

On preview: The Airport Express doesn't need to understand DOCSIS 3.0; that all happens on the line side of the cable modem. If Comcast's modem firmware update has ballsed up the ethernet link - e.g. link speed / duplex / mode negotiation - I'd expect to be seeing a lot more problems reported around the internets. The Airport range really is very good when it comes to dealing with poor ethernet hardware, dodgy DHCP servers & clients, etc. But, to be sure - what address is reported at the bottom of the "Summary" screen?

The only thing I'm not sure of is Comcast's configuration - does the modem provide a straight IP address via ethernet, or does it require a login (e.g. PPPoE) via client software or router? I'd guess the former in this day and age, but being well outside of Comcast's coverage area I dunno. Either way, the setting for that is under "Internet" -> "Internet Connection" -> "Connect Using" in the Airport's configuration screen.
posted by Pinback at 10:55 PM on December 29, 2010

I would second nulledge's advice. When Comcast is setup it is bound to the MAC address of the initial computer. Some modems and gateways can clone this address, it looks like the Airport can not. See Apple's KB on this. They advise asking Comcast to change the MAC id on their side to the one that you give them off the Airport. It is possible that the previous Airport was registered with their system.
posted by Disco Moo at 10:59 PM on December 29, 2010 [1 favorite]

I've found that it takes considerably longer than five minutes for the modem to recognize the new computer/router that it's plugged into. Although it seems these things should be instantaneous, I've found that it may take upwards of 10-20 minutes. I did the same things as you did, unplugging things and actively debugging. It was only after I gave up and left it plugged in that it started to work.
posted by meowzilla at 12:53 AM on December 30, 2010

Comcast has been upgrading their local networks to handle DOCSIS 3.0

It is the cable modem that handles DOCSIS, not the router. The router shouldn't matter, assuming it is otherwise working correctly.
posted by Blazecock Pileon at 4:04 AM on December 30, 2010 [1 favorite]

I would investigate two things.

1. Try setting your AirPort to use Google's DNS servers at 8.8.8.8 and 8.8.4.4 . Comcast's DNS servers have been driving people nuts recently.

2. When in doubt, try a different network cable between the AirPort and the cable modem.
posted by advicepig at 7:22 AM on December 30, 2010 [1 favorite]

I'm with those that worry about Mac address linking. Every router I've gotten on Comcast has required me to spoof my PC's Mac address (usually the router itself has a setting for this in its config so you don't have to call Comcast)
posted by bitdamaged at 7:31 AM on December 30, 2010

Alterscape's advice is probably the most correct. But there is a dance that needs to be done, it doesn't seem to work right ever, on the first try. This has reliably worked for me in the past:

1. Do like you did: power up Airport completely, then plug in modem and power it up.

2. That won't work, so then you power up the modem completely, and then plug in the Airport, and power it up.

3. That won't work either, and then you do set 1 again and it does.

There are two things going on: the MAC address conflict is one, but also the router has a DHCP lease from the cable modem that it needs to release and renew.
posted by gjc at 10:09 AM on December 30, 2010

It seems it was a MAC address issue.

After much trial and error on my side, I finally called Comcast back. They reset my modem and I then unplugged and replugged my Airport Express. Now everything seems to be working fine.
posted by jaybeans at 4:15 PM on December 30, 2010