I knew a guy once that did broadband installs and he said that the ISP takes the MAC address from the customer's modem and inputs it into a script on the ISP's
authentication server. Anyway he said this script allows them to turn off your internet or turn it on just by changing a value in the script. I always was led to beleive that a MAC address is only used inside of a LAN, not on a WAN so I'm ...