You only need their Comcast Business modem if you have static IP's because they route them using RIP with a password that is set inside the cable modem with the

You only need their Comcast Business modem if you have static IP's because they route them using RIP with a password that is set inside the cable modem with their custom software. If you don't have a static IP with Comcast Business it makes it awfully hard to run a mail server, but then you can indeed use your own cable modem.