Man i got to tell you.... i reserched this for hours....
ok

First things first....
you need a gamecube Broadband Adapter OR a nintendo modem adapter

WARNING The Nintendo gamecube was not ment to hook up with lan

but... i've heard rumors that if you hook up your game cube to your computer and go to warppipe.com you can play people over the net
check these comments to check my statement:
http://www.amazon.com/gp/product/B00007DWBL/103-1110638-3971862?v=glance&n=468642

you'll see near the bottom a coment named
Broadband ROCKS!! MK: DD DOES WORK EVERYONE, July 28, 2004
A Kid's Review

if you have more questions just pm me... 8)

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yep... I'm back...

Yah but did you see the review?

http://cubeonline.warppipe.com/

it looks like this can work...
I might try to get the gamecube broadband adapter for myself 8)

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yep... I'm back...