I would ask the ISP.
If you're on a point to point connection, you could have a subnet mask of 255.255.255.248 for example. On a broadband connection, you may actually be a part of a larger subnet with 6 addresses assigned to you.
You're ISP should provide you with your addresses, subnet mask, and gateway information.
BierHunter
CNE, MCSE, CCNP