From: ChromaticAngel | #005
you don't need to forward ports to connect to XBox Live. I have never forwarded ports on any of my 360s and every single one of them passes the connection test with flying colors and I have no problem using any XBL features.
I never said you had to forward ports.
If he is forwarding ports, it explains why one adapter connects and the other doesn't. Furthermore, if he's forwarding ports and he has a PS3, he's probably forwarding port 80 to his PS3 because that's what the guides say to do, and that could be the reason he's not able to connect with his 360 though it wouldn't explain why one adapter connects and another doesn't. .
In your case, you say you have multiple 360 so of course you don't forward. You can't forward to more than one at a time.
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Fix your NAT: http://www.freewebs.com/haximo/ports.htm