Assuming you are using the Xfinity router for WiFi and that you have a local area network set up, your IPs on your devices should be in the range of 192.168.x.x, or 10.x.x.x. Whichever it is, the router has a DNS function at the 1.1 address, so 192.168.1.1, or 10.x.1.1 depending on your LAN range the DHCP server is using to assign numbers. That is the "default" but you can also add others, including the ones given by the other folks who have posted. The system will use the DNS sources in the order that they show in the System Preferences box where you enter the addresses for DNS.
Why the 1.1? Because most system reserve that address for the router itself, so basically you are querying the router for DNS service and it passes the request to the DNS address it has been told to use by the Xfinity network to which it belongs. Your request goes up the line until it has hit a DNS service that has an address for that URL, at which time the results are passed back down the chain, eventually reaching your system and you are on your way.