Originally Posted by Blender
The address is likely to be 192.168.0.1 or 192.168.1.0
The two most common I've seen are 192.168.1.1 (about 90% of the time) or 10.0.0.2 (about 5%) and any random (valid) IP address making up the rest.
The suggestion to get the manual is probably the best. The first thing I'd do, however, is turn the router on (doesn't have to be connected) and press the "Reset" button with a paper clip or pen, this will make following the manual instructions much easier.