Bear in mind that the router is really not "free". In further reading in the link that James supplied you will see that it remains to be the property of Comcast and must be returned if you cancel your service in future. There is a $ 9.95 shippling charge. What it is "free" of is a monthly rental fee. I think it is a good deal !
(The thread I referred to would come up at the top of the search results, if not for this more recent one. And not surprisingly that thread contains pretty much the same info, from the same responders as here.)
LOL, EG. I hadn't even noticed the kudos. My first post was a roundabout way of suggesting the OP might not have put much effort into their "search"--since even the sometimes funky search feature here turns up precisely the info they were looking for.
Sometimes I think we make things entirely too easy for people. Even as we wonder whether folks ever really try to find answers for themselves when we know they already exist here. As you know, sometimes I expect the horse to put in a little effort to get to the water.