Very interesting bug I thought I’d document somewhere. My current home ISP service is AT&T UVerse and I recently 
had it serviced because of some issues. I ended getting a whole new router – the Pace 5268AC. This was very exciting because all of my other equipment is no faster the N speed — this router gives me wireless AC. Woot! Although the router is so much better than what I had, still had some issues with my AirPort Express.
I had setup two v1 AirPort Express devices to some powered speakers. One that I kept outside. They stop working well over a year ago and I thought it was because of a bad connection to the older router. Still had the bad connection issue after the new router. So it must be the AirPort Express. So I figure time for another upgrade. I jumped on eBay and picked up a used late model AirPort Expess.
It arrived in the mail today and I of course couldn’t wait to set it up. This is where things get interesting.
If you use the Airport Utility application to configure the Airport Express, you will not be given the option to configure the 
device as a wireless client on the existing wireless network. You must first set it up in a new network. Obviously it is much easier to configure the device if you plug in an Ethernet cable to the Airport Express. Once the device is back up, you can then configure it to be a wireless client on the existing network. So I basically named the device, entered my wireless name, etc.
Once the AirPort Express restarted – my computer and most of my network couldn’t not access the Internet. Somehow, some way, the AirPort was seriously screwing up the routing on the network. I could access anything that was on the LAN side by using IP addresses. As I use an external DNS server for DNS, I couldn’t access anything by name. I also couldn’t access the router’s LAN port to see what issue the router would be reporting.
In order to get everything back online, I had to unplug theater net cable from the AirPort Express and break the connection between my router and the down stream switch the AirPort was plugged into. Once I reconnected the switch, we were back online.
Fortunately, the AirPort Express was still connected. I moved it outside and hooked it up to my speakers and it works like a champ! It’s going to be a nice relaxing weekend with poolside music.
-JT