Blinking Amber light on Airport Ext after modem replacemet, but internet still works

I recently installed a new At&t DSL modem (Motorola Brand) and connected it to my AirPort Extreme router. I can get to the Internet over the wireless connection on my Mac, but the router is still flashing an amber light instead of the normal green and the AirPort Utility has a message about a “Double NAT.” What I have read online tells me to change the Airport to "Bridge" mode. I did that, but then I loose the wireless connection. I also get an error message that tells me "Another device on the network is using your computer's IP Address"

How do I fix the blinking amber light????

Also, I'm not very computer lingo savvy, so please speak in elementary terms
Thank you in advance for any suggestions!!!

15" MacBook Pro, running OSX 10.8.2

Well let's understand the lingo before we figure out what's going on.

NAT stands for Network Address Translation and is a a way of "bridging" two networks together so that the packets can flow. So the Internet operates on the public IP range, and your ISP gives you one of those for the DSL modem. However, you want to have 5 devices access the Internet using this one IP address. Well you put a router between the devices and the DSL modem. The router gets the public IP address assigned by the ISP and then uses a DHCP (Dynamic Host Control Protocol) to grant internal IP addresses to each device that wants to access the Internet.

The router also keeps table of which IP addresses have been giving out and uses this to determine where a packet came from and where it should go back. This is known as the Routing Table.

Now, suppose you want to run a Virtual Machine on your Mac with Linux/Windows on it. You want to give the guest OS access to the Internet. The quickest method is to configure the VM with a NAT'ed address against the IP address you were given by the router. So when the guest OS accesses the Internet, to the router it actually looks like your Mac and routing continues to happen.

Now in some cases you want the guest OS to be able to access the Internet on it's own internal IP address because you want someone else on the network to access the guest OS like it was a real machine on the network. Here, you configure the VM to use a bridged network adapter. This will make the guest OS request a IP address from the router just like it was a real machine..

So, coming back to your problems. When you see a message about Double NAT. You basically have 2 devices doing the same work.

Is your AT&T DSL Modem just a modem, or is it also a router (with Wireless). If it's just a basic modem, then you shouldn't be seeing this situation. If it's more than that, you will want to disable all of the router capability on the modem since the Airport Extreme is doing that work for you.

My Modem is just a modem. It isn't wireless.

Do you have any computers hooked up to the ethernet ports on the modem.

Assuming that your Airport Extreme Router (AE) was working fine with your old cable modem…after you received your new cable modem…did you install everything (the cables) exactly the same way as the old setup?

If so…then sometimes the order in which things are attached is important. For example…many times when first setting up the new cable modem...it's better to let it completely initialize before attaching the AE.

So what I would suggest is…first verify that all cables are in the proper ports for the cable modem & AE. If they are…and the yellow light is still blinking on the AE…then I would disconnect the AE from the cable modem…then reconnect it (this should most likely be the ethernet cable coming from the cable modem).

- Nick

no - the only cables hooked to the modem are the power cord, the DSL line in (from phone jack) and the ethernet cable going out to the router. This entire config. worked great until I had to replace my old (faulty) modem with a new one. I can get it to work without the router (hardwiring it straight into my laptop) but then I have no wireless signal. This new model modem is different from the old one. The old one was a Motorola 2210-02-1022. This new one is a Motorola 3360.

Nick - yes everything was working fine before the old modem went haywire. I installed the new modem + it's equipment and allowed it to initialize before attaching the AE. I found this article, but changing the AE to bridge mode didn't help

Q&A: A Fix for a Blinking Router - NYTimes.com

Q&A: A Fix for a Blinking Router
By J.D. BIERSDORFER

Q.

I recently installed a replacement DSL modem and connected it to my AirPort Extreme router. I can get to the Internet over the wireless connection on my Mac, but the router is flashing a yellow light instead of the normal green and the AirPort Utility has a message about a “Double NAT.” Is there something wrong with the new modem?

A.

The “Double NAT” error is not uncommon if you have recently switched to a new DSL modem. NAT stands for Network Address Translation. The router uses this technology to keep one public Internet Protocol address from the DSL modem for itself and assign private I.P. addresses out to the devices connected to its own network.

This scenario usually works quite fine unless there is another device in the mix that is also trying to do the Network Address Translation chore — and then conflicts can occur. You still may be able to reach the Internet on your devices connected to the home network, but the AirPort Extreme router will flash its yellow light as an alert and some network-based features like Apple’s Back to My Mac service will not work properly.

Since you have recently changed DSL modems, the new model is probably now providing its own NAT services, whereas the old modem did not. Since the AirPort Extreme is also doing its own NAT work, you get the Double NAT error message.

One way to correct the problem is to turn off the NAT service on either the modem or the router, which puts it in “bridge” mode. Your Internet service provider may not want you messing around with the modem’s settings, so you may want to call the technical support line and inquire before attempting anything.

Turning off the NAT service may be easier to do on the AirPort router. To do so, open the AirPort Utility program. In the latest version of the utility for Mac OS X 10.7, click the image of the AirPort Extreme. A box pops up showing technical information and all the devices connected to the router. Click the Edit button and then click the Network tab. Under Router Mode, select “Off (Bridge Mode)” and click the Update button.

Once the NAT service has been turned off in either the modem or the router, the AirPort Extreme should stop flashing yellow and return to its happy solid green light. Apple has more information on setting up the AirPort Extreme as a bridge here. (The company has instructions for putting the router in bridge mode on older versions of Mac OS X with earlier versions of the AirPort Utility at the bottom of the page here.)

Hi All! Thank you for the help and suggestions.....I have an IT friend that I finally got in touch with, and it looks like we fixed the problem.

We logged onto the att modem connection site (192.168.1.254) and changed a setting from "Use private IP address" to "Use Public IP address" After the modem reset itself and I power-cycled the router, BINGO! Green light.....go figure.
Geez - I just wish att tech support could have told me that DAYS ago......