Usually a router listens on its own internal IP address (it would be the default gateway of your connection). You can check this by opening a command prompt and running 'ipconfig'. You can then look for your connection and see the default gateway (depending on your IP Address scheme, it might be 192.168.1.1). Running XAMPP should have no effect on that IP but without knowing anything about your router/modem, your setup, or how you are trying to access things it is really hard to say one way or the other.
How are you trying to access your Modem/Router (Internal IP/External IP/other method)?