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As said by Aragorn, every device that connects the cable via a Set Top Box (STB) has to have the mac address registered. That should only be a one time process. If you change the connection from one PC to a different PC, the STB will not realise this unless the STB itself is rebooted. Once the STB is rebooted, the first time a new PC attempts to connect to the internet, it should automatically be detected and the user is pointed at the ntl registration screen (customer will need their account details to complete this).
Reboting every time the user needs to swap between pc and laptop, especially if that is frequent is a PITA. A simple router gets round that (altough the router too would need to register with the STB unless a mac addy is cloned into it from a known computer).