@Sszakats wrote: My xfi router is connected to my cable modem via Cisco cable. Should I also have an Ethernet connection to the cable modem as well? How can coax cable support gigabit speeds?
Your question isn't clear. What is the model number of the "xfi router" and of the "cable modem"? What kind of cable is the "Cisco cable"?
If you are renting from Comcast, you probably have an all-in-one modem/router/Wifi gateway device. If you purchased your equipment it could either be a similar all-in-one gateway, or a separate modem and router. In either case, you connect a coax cable from the Comcast drop to the modem or gateway, and Cat-5e or better Ethernet cable from that device to the rest of the wired devices on your network.
Coax cable goes to bed early, gets up early, has Wheaties for breakfast, and works out every day.