If IP Address is failing, ensure you have DHCP running on your network. Most people will have this enable on their router.

DHCP (Dynamic Host Configuration Protocol) ensures that devices plugged onto the network obtain IP Addressing automagically.

You can plug your Xbox into your comp upstairs, however you'll need an additional network card in the PC, and you'll need to bridge the connections i believe - This would be an uncommon setup.

It would be easiser to buy a 5 port switch or something, plug the ethernet cable coming up from downstairs into the switch, and plug both the Xbox and PC into the switch too.

5 port switch = $29 or somethin.

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[SIGPIC][/SIGPIC]
Should have bought an STI