Are you using a direct connection, or are you connecting to a router? <br /> Was your service configured with that Mac laptop or a different computer? Comcast recognizes one MAC address, for the device that was being used during the configuration of your services with them. If you are connecting directly to the broadband modem, you can't just plug any computer into the modem and expect to be able to connect - the MAC address of the network adapter installed in that computer won't be recognized. If you are using a router behind the modem that shouldn't be a problem, Comcast should recognize the MAC address of the router if that was connected when service was setup initially.