When you get internet service from Comcast, they rent you a cable modem. This costs around $10/month. The nice thing is that if you ever have trouble with the modem, you just replace it at no additional cost. But if you’re willing to put in a little extra effort, you can ditch that monthly fee.
Start at the official Comcast help page. They walk you through the steps of figuring out what kind of modem you should get. There are a lot of options and your specific choice depends on whether you have your telephone through Comcast too or whether you want to have your modem and router all in one device or in two separate devices.
When I buy a modem, I try to get one with the latest tech so that I don’t need to upgrade it very soon to get faster service. Currently, the top of the line units have DOCSIS 3.1. That supports some pretty crazy speeds, but as long as you get DOCSIS 3.0 you should be fine. Being a huge nerd, I like to have a separate modem and a separate router. I don’t have Comcast telephone service. I bought our current modem back in 2012 for $89.99 from Newegg and it is a Linksys DPC3008-CC R. It looks like it’s only $37 now.
I don’t remember the details of how you actually do the replacement once you have the new unit in hand. I think you just plug in the new model and then call Comcast support. They’ll ask you for the “MAC Address” of your new unit which is printed on the device somewhere. After that you can ask them how to return your old modem.
In my case, I broke even on the cost of the modem in 9 months and ever since then I’ve been saving $10/month. If you’re willing to get a little nerdy, it’s an easy way to save some money.