By default, most ISPs don't let you have more than one public IP address, or if they do, they complain because you're using more than one computer on the network, and once upon a time, they were silly enough to believe that you'd pay twice to use broadband on two different computers.

AT&T may be blocking inbound port requests. If you're using DHCP, then what happens is that one game is running a server, and when the other machine tries to connect to it, it goes out to the Internet, and back in. Effectively, you're running a game server.

Your network setup isn't exactly ideal either, and it makes you vulnerable to network attacks. You should definitely consider getting a firewall/router. A 4-port Linksys router costs $99 at Office Depot. You could also get a single-port router cheaper, and plug it into your hub, but be sure you find one that performs NAT, letting you get on the network with more than one computer. Otherwise, it won't work.

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"And knowing is half the battle!"