these are technically regular modems ... motorola surfboard was the first our company used now we use some cisco router/modem combo... the firmware is still written by the original manufacturer but its customized by the isp... like cell phones...
sites for the good isps will list products that will be accepted (compatibility) on their service if you dont want to use the isps modem...
however comcasts website needs a 'sniff' tho since i dont know of their policies... i dont know if comcast forces rental....