Well, if this is your local network, why not to get to the computers which are plugged in your LAN (your router), and check their IPs?
(If someone wouldn't know, to do that open up: "run" window (start->run), type in cmd, press enter. Now type in: ipconfig. You'll get much infos, but your local ip usually starts with 192.168. and ends with 100 or up.)