What is the ip adress of my modem ,don't most modems heve the same ip adress?

Are you using dial up, and asking about an actual modem in your machine? More than likely you mean a cable modem, in which case an easy way to tell is to open your command prompt (click start, click run, type 'cmd' and hit enter) and then type 'ipconfig /all' in the black window that pops up. Your modem's address should be next to the label "Default Gateway" . This is a best guess answer assuming you are an average user at home connected directly to your cable modem. Hope it helps.

your thinking of the IP that you go to, to configure your modem's settings

Modems don't have IP addresses. Routers do. If you have a non-routing modem, the computer draw an IP address from your provider. If the modem is also a router, its IP address depends on its make (and possibly, the model number). That would be an internal IP address - its external IP address is one it draws from your provider. Routers have internal IP addresses in the 10, 172 or 192 address range.