To sign in to your router's administration panel, you need the router's IP address. Typically, routers use 192.168.1.* or 192.168.2.1 as the IP address. In cases where they don't use this address, however, the Command Prompt's ipconfig command reveals it.
For Windows 8, press Windows-X to open the Power User menu, then select Command Prompt. For Windows 7, press Windows-R, enter cmd in the Run field and click OK.
In the Command Prompt, type ipconfig and press Enter. The heading you need depends on how you're connected to the router: Wireless LAN Adapter Wi-Fi if you're using Wi-Fi, Ethernet Adapter Ethernet if you're using a wired connection. The number listed as the Default Gateway is the router's IP address.