The majority of the time, we have no idea what the IP addresses of our devices are. You don't need to know that your phone has an IP address of 192.168.1.12 on your local network in order to use it.
I'm currently using a WRT54G router with Tomato firmware and I have a simple residential dynamic ip address cable modem. The cable modem is in "bridged" mode and the router is set to use DHCP to get ...