I install my new Motorola SB6121 Cable Modem today . the cable connection and bootup of the modem complete without a hitch and I contact Xfinity Cable -lrb- Comcast -rrb- and speak with a internet specialist who take my HFC MAC ID number and my customer serial number off the box . after reboot the modem and router -lrb- wrt54gl -rrb- , I bring up the computer . at first I could not connect to the internet , but after I change my lan address it come up okay . the reason it do not come up right away be that the cable guy reset my router and the setting be all set back to the default . fortunately I have save my router setting prior to contact xfinity and when I restore they back to the setting I have , everything work perfectly . so be sure if you be use a router that you save you favorite setting before you do this modem change . it can be a real pain try to remember all the setting in the router that you may have change over the year . by the way , I never connect to the computer first . Modem , Router and Computer be all connected initially but be boot in that order . so far everything now be work fine and I hope that continue . a bit too early to say how this will work out over time , but at least it be save I $ 7 a month for the time be . by the way , the Xfinity guy be great , polite and very knowledgeable . good luck . UPDATE : it be be about a month since I write the above review and since that time the modem have work flawlessly . I know some folk say there be have run very warm , but I do not find that to be true . my modem be only slightly warm to the touch , but a whole lot cooler than a lot of my other electronic device . as far as I be concerned , this be a excellent modem and be work well with my xfinity -lrb- Comcast -rrb- account . download speed 20mb and upload speed 4mb . ca not beat that in my book . sure beat the lousy dsl service I previously have .  Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem