Note that the majority of ISPs (at least in the US) will allow you to register a new MAC address if you request it. It's this new MAC address on a new modem and/or router that would have to be registered with them to allow you to use a different one than what they provide. Usually, just call them and ask to register a new MAC address because your current router/modem doesn't work and they'll do it in a matter of minutes.
