Try IPCONFIG.EXE

If it's a router, usually it will provide DHCP service.
Try c:\>ipconfig.exe
it will tell you something like:
Windows IP Configuration


Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.2.102
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.2.1

The default gateway is the one.

Otherwise, usually the first three digit is the same, for example, you ip will be like 192.168.2.xxx, then try to use ping to ping the 192.168.2.1

Just in case it's not 1, then you might want to do a simple loop to ping from 1 to 255.
C:\>for /L %i in (0 1 255) do ping 192.168.2.%i

Also, most of the time, if you go to that http://192.168.2.1 (the router address), you will be able to config the router.