The modem's IP is assigned by your ISP and doesn't align with home router IP's. If everyone's modem were in the 192.168.x.x range they'd all be interfearing with one another and hackers would have a field day.
No Bald, you don't quite understand. The Modems usually work like a quasi-router, with both a public AND a private IP address. You can actually access most modems by putting in 192.168.1.1 or whatever depending on the brand.