So when your entire neighborhood is NAT'd and you want to use XBox Live you'll, what, call up your ISP and ask nicely to have the right ports forwarded to you?
NAT works in an office environment because most offices aren't doing anything more than web and email. Home usage scenarios are significantly broader and harder to keep working behind carrier grade NAT.
Presumably (I haven't got one) the XBoxes can use UPnP to negotiate external ports with the home router, which, unlike the ISP's router, can trust the devices on its network.
NAT works in an office environment because most offices aren't doing anything more than web and email. Home usage scenarios are significantly broader and harder to keep working behind carrier grade NAT.