Just to alert everyone about a very frustrating experience I had in which I wasted a day of my life.
I have DSL through Earthlink using a Netopia modem which they provided. I have that directly connected to my Airport basestation. My airport network is extended using an Airport Express.
Two nights ago, I updated the firmware on both Airport units to the latest version released (I think) 1/3/06. The versions are Basestation 5.7 and Express 6.3. (Actually, I upgraded first to Basestation verion 5.6 which started this problem then to 5.7 in an attempt to fix)
This firmware update completely screwed up my network. I could no longer connect via dsl modem to the internet. I went through the usual troubleshooting and reset both units and started fresh with new configurations. I spent the next 6 hours or so trying things on my own and finally called Earthlink tech support and Apple tech support the next morning. Over the next 8 hours or so we tried and retried various iterations of the same theme. The Airport units would not communicate with the dsl modem and I kept getting the message "Looking for PPPoE host..." message as the basestation would try to establish communications.
I knew the dsl modem worked because I could establish normal communications when I connected directly to my ethernet port. I knew that the airport units worked because I could create a separate local network plus I could configure both units wirelessly as usual. And I could use the Basestation modem to establish internet connection with Earthlink. I reset the modem to clear any stored old IP address. I also configured the Express as the basestation with no luck.
Finally out of desperation, I went back to the Apple downloads website and downloaded older versions of the Airport firmware and installed v5.4 (which I think was the version prior to all this trouble) onto my basestation.
Problem solved. I spent the rest of the evening getting everything back to the way it was.
I called Apple and Earthlink to let them know of the problem and they agreed that this was most likely a compatibility issue between the latest firmware update and the dsl modem.
Has anyone else had this problem with other DSL/Cable providers or modems?

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