This installation is very simple. Remove the COMCAST modem. Someone in another review said you need to remove your router temporarily for this installation, so I did that and connected the CM100 directly to the ethernet connector on the computer. Once you did that, you need to call COMCAST and give them the S/N and the MAC Address, both of which are printed on the bottom of the Linksys CM100 unit. They enter your new info in their system and you are up and running. It really is as simple as that.
Obviously the Comcast modem won't work after that?

To get them both to work, you would need to clone them to have the same data.