Also, the OP must ensure that the modem that he buys conforms to the standard that this Comcast franchise uses. This will be either DOCSIS 2.0 or DOCSIS 3.0. The sales staff at the store where the modem is purchased will help him with this.
After his purchase, the OP must telephone his local Comcast office and give them the MAC address and the serial number of the modem. This information should be clearly labeled on the case of the modem.