It's a number in the cable modem itself.
If yours worked, you'll have to pay up.
I live in an apt complex where internet is provided along with all utilities however the internet has terrible speeds(broken backbone). one of my roommates had a cable modem in his room for better internet for a while then he moved to another apartment in the same complex. he connected up his modem(cable modem comcast or charter) and got internet through it with out calling the cable company and transferring his account. i figure this is possible b/c our apartment complex gets one cable connection and somehow routes it to the rest of the apartments somehow. MY QUESTION IS: do these cable companies connect to his mac address in his modem or are they sent to the cable places that pay the bills (ie the apt complex cable) and could i buy my own cable modem and connect up to the service w/o paying an extra monthly fee?