By default Xfinity Gateway and most of the other routers use DHCP protocol to assign addresses automatically to each device on a network. It means when device boots it automatically requests an IP address from DHCP server (unless there is a dedicated DHCP server running on a network, gateway will play DHCP server role). IP address is assigned from a pool of available network addresses in a way to ensure that there are no IP address collisions.
But there are cases when you may want to have a fixed IP address assigned to a device. Such IP address is called static IP address. Device with static IP address will always be identified with it on a network. This is very useful when setting up port forwarding (assigning network port to a specific IP address so devices on the network could be accessed externally). But it also adds some complexities like making sure that there are no IP address collisions on a network. Also any reconfiguration of the network may cause statis IP address assignment to fail. Thus static IP configuration is better performed by service technician.
If you need to assign static IP address to your Windows PC you need perform following steps: