huh? thats not what comcast told me. i spoke to them and they said that when you register your modem, you register your mac address, so when you do that they assign your mac address and ip.
in that theory if you change your modem's mac address you won't get a connection, because that mac is not registered to comcast.
but my friend's network is the same as my network.
we both have comcast cable connected to a router, and he has a netgear router (4 ports, wireless) and i have a d link router (4 ports, wireless)