Windows XP or Vista:
Type ‘cmd’ and press enter
Type ‘ipconfig’ and press enter
Look for the default gateway — that the ip address of the router
Mac OS X:
From the Apple menu, select System Preferences… .
In System Preferences, from the View menu, select Network.
Next to “Show:”, select the appropriate port. For example, choose Built-in Ethernet for broadband connections, AirPort for wireless, or Internal Modem for dial-up.
Click the TCP/IP tab. The number next to “Router:” is your default gateway.
Mac OS 9:
From the Apple menu, select Control Panels, and then TCP/IP.
In the TCP/IP window, the number next to “Router address:” is your default gateway.