If you just want to know what the IP address of a computer on a network is, just go to the Start menu, select Run and type in CMD to open a command prompt window. Then type IPCONFIG in the window and hit enter. It will show you your IP address for that computer.
If you want to know what your Public IP address is go to
www.whatismyip.com and it will show you the IP address assigned to your modem at that moment if you have a dynaminc address from your ISP or it will show you your permanent IP address if you have a static address from your IP.