Q&A: A Fix for a Blinking Router 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? 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.