(Before you start...You may find the videos at the bottom of this page helpful.)
Below is a list of Comcast/Xfinity approved retail modems and gateways that can be used in lieu of rental equipment. This is not an official list but is compiled and maintained by the community. Please visit https://mydeviceinfo.xfinity.com to verify that a specific device will work with your service. If you find an error or information that needs to be updated, please feel free to send me a private message. Thank you!
Modem: This is the basic device you need to connect your home network to Comcast. A modem will provide a single ethernet connection. Most people connect the modem to a router in order to have more than one wired device and WiFi service. Some modems provide voice capabilities.
Gateway: Many people don't want to deal with a separate modem and router. A gateway device is a combination modem and router. They typically allow four wired connections and provide WiFi. Some gateway devices also provide voice capabilities.
Channels: These are communication paths. The more channels the better. This is expressed as downstream channels x upstream channels. So a device that is labeled "32x8" has 32 downstream channels and 8 upstream channels. Higher speed tiers require modems with more channels. Each channel can handle ~25Mbps, therefore an eight channel modem would be capable of ~200Mbps.
To find your speed tier visit https://mydeviceinfo.xfinity.com. After you log into your account, your speed tier will be displayed on the left-hand side of your screen. Alternatively, you can view the speed tier to which you are subscribed here. Sign in and scroll down to the data usage graph area.
Netgear CBR40 32x8 gateway (Orbi)
DOCSIS 3.1 Devices (required for gigabit speeds)
Motorola MB8600 32x8 modem (DOCSIS 3.1)
Arris SB8200 32x8 modem (DOCSIS 3.1)
Netgear CM1000-100NAS 32x8 modem (DOCSIS 3.1)
Netgear C7800 32x8 gateway (DOCSIS 3.1)
Voice Capable Devices
Arris TG862G 8x4 gateway telephony (Same as TG862G-CT, TG862G/CT)
Last update 9/12/18 @Nerdburg
Thanks to @darkangelic for proofreading.