A network like yours uses a process called "network address translation" to allow multiple devices to all share the same public IP address. That is the address people on the internet see for all your internal devices.
That public IP address is assigned by Comcast each time the modem connects to their system. Inside thier system (beyond your control) they have decided what happens when that connection is broken. Thier system "reserves" that address for your modem for a period of time (determined by them) from 0 seconds to forever. It's called the lease.
You may be able to affect your public address by unplugging your equipment and waiting for a while and then reconnect it. If your "lease" has expired the address will change... but you can't choose to what. They do.
If you are talkin about your inside addresses, changing them is a multi step process. First, log in to the router and change the assigned range for the DHCP ( like 192.168.1.10- 192.168.1.100). Then re-assign the local address of the router to a static address in that subnet, but outside the DHCP range (like 192.168.1.5). When that change applies, you will lose contact with the router.
Boot your workstation so it picks up the new DHCP setup and you should be able to see the router on it's new address. Boot your other workstations so they also pick up the new setup.
Then go to each of your devices and make sure that they are set to get thier address automatically. Then reboot them.