[car1son]

KrecJ -
It's expected your Airport and iMac will have different Ethernet/MAC addresses. The datacom industry has gone to some effort to ensure that all Ethernet addresses everywhere are unique. (The exceptions are a few specialty devices specifically made to "clone" or "spoof" an address - generally for such things as routers (e.g. my LinkSys), test equipment and spy agencies.
It's not terribly common in DSL, more so in Cable broadband because of the nature of the wiring, for the Ethernet/MAC address to be locked to the account. "MAC Authentication"
As to me not getting the same eMail - well, first all DSL is local. There really is no Earthlink DSL monolith, just a patchwork of local DSL hubs and spokes from various providers. My services runs through Qwest and Covad (in fact, if I look up my assigned IP address at ARIN, it always shows as belonging to Covad. (For still more variety, the Earthlink backbone I'm fed into is actually part of the old MindSpring network Earthlink acquired five years ago - a topological anomaly that at least once a year causes someone at Earthlink to turn something off that's still needed to keep me connected - but that's just my problem!)

(If you visit DSL Reports and feed in your ZIP code you might find out more about your specific network. Presumably BellSouth is involved, at least with the wires.)
Finally, it's very hard to guess exactly what Earthlink's eMail to you is saying. It's possible they have previously allowed you to allocate more than one IP address from their hub and are now taking that away; in that case, they are perhaps just telling you that you need a Router where before you didn't (but of course they wouldn't point you at any specific router solution except the one they sell.) Or, they might be switching to MAC authentication (in which case you can just have them use the Ethernet/MAC address of your Airport (WAN) port.
It is theoretically possible that ISPs could detect the use of Routers and NATs running more than one computer and block it to force customers to purchase solutions from them, but to the best of my knowledge none in the US has done so. (I'm sure every time they look at LinkSys's sales figures they drool and think about it, though.)

[Nobody]

Car1son, Velvet,
Well, June 24, 2003 has come and gone. I do not have any problems accessing Eathlink from two PCs from behind the AirPort Base. If Earthlink is using "MAC Authentication", how would this affect things if I were, for example, to upgrade to an AirPort Extreme at some point?
KrecJ

[kworley]

Uh... I've been using Earthlink's gateway equipment with OS-X computers for over a year to run my home network with no problems. I have a G3 Pismo laptop and an iBook connected on wireless, and an eMac on ethernet. I think the issue is the "setup wizard" software (which I don't use anyway) that may not be quite compatible with OS-X, but as for actually using the equipment, there's no problem.
Katrina