There was an error connecting to the ATT network

No kidding! Yes, the LAN is on in AirPort and when you enter 192.168.1.254 (the router IP address) a message pops up stating there is no internet connection. I'm not sure how you're defining "Interface" but it is not wired to anything and I cannot detect any other wireless LANs within this geographical area.

Interfaces would be Wifi/Airport, Ethernet. & such.

Sorry, no idea where to go from here. 😟

Is it possible that it somehow got infected with a virus? Would reinstalling the system software help?

Hmmm,

See if you might have this malware redirecting DNS queries...

http://macmegasite.com/node/3924

http://www.ehow.com/how_2128387_remove-osxrspluga-trojan-horse-mac.html

How to fix...

http://www.macosxhints.com/article.php?story=20071031114140862

Known DNSChanger address ranges. Source: dcwg.org

http://krebsonsecurity.com/2012/03/court-4-more-months-for-dnschanger-infected-p cs/

Okay, I went to all four links. The first one said "Page not found." I used the instructions in the 2nd and 3rd link and followed them on the affected iMac. It checked out okay with no apparent virus in the computer. I wonder if the problem is with the modem blocking it. I can't figure out how to manually configure it to allow it access.

See if this helps on checking the router...

http://techpaul.wordpress.com/2007/07/31/more-internet-safety-use-your-router-fo r-access-control/

Thanks for the tip on the dead link.

Thank you for the tip. However, the Netgear 7550 router is not what you would call "user friendly" in my opinion. I've tried entering the Mac address manually and - you guessed it - no changes. I think I've just about exhausted all of my options at this point, short of reinstalling OS. Before I do this, do you have any other suggestions?

I'm pretty much out of ideas, sorry.

The computer works fine with ethernet connected. My logic tells me that perhaps it is something mya be wrong with the AirPort card. Does that sound logical to you?

Yes,ither the Airport card, or Airport Software/OS.

I use these even where I have working Airport cards...

http://eshop.macsales.com/item/NewerTech/MXP2802GU2/

Thank you! What I will do next is reinstall OS 10.4. Then connect via ethernet and update all software and try again. If that does not work, then I will try a new AP card. You're a genius. 🙂