How do they know?
The only way they can know is by the NIC address of the first network device connected to the cable modem. Routers can be told to clone the address of the computer that used to be connected to the modem.
I suppose I got Earthlink DSL because they don't play that game. Their tech support even helped me set their modem to an undocumented configuration designed to work with a router.
I suspect that you mean MAC address??
Actually it's the MAC address of the NIC that is registered.
Go ahead call me a geek.