Depends on what OS you are using and, if using NT or similar, you have administrative rights to make changes on your computer.
There is an easier way than what has already been posted to track your IP address if you are using 98 and, I believe, ME. I am not at all familiar with XP yet.
On the start menu, choose run, and type in "winipcfg." This will pull up your IP information. There should be a drop-down bar the first thing you see will be "PPP Adapter," and if you pull down the menu you should see the name of the device with which you're accessing the internet/network. You will want to select that device, and then you should see your IP address, along with your subnet mask and default gateway numbers. More advanced information is also available.
With NT and 2000 you probably have to enter those commands at the DOS prompt as per the previous post in this thread. As I mentioned, if you do not have administrative rights you may not be able to view or change your own IP address.
The IP number... is always the same if your ISP provides a "static" IP address. If not, then it should be different every time you dial up or otherwise connect. Usually, static IP's are only available through broadband, either networks or high-speed ISP's such as cable, dsl, fixed wireless, etc.
If you are attempting to route something directly to your IP, and it is not static, you will need to renew your routing every time you renew your internet/network connection.
Hope that helps.
- Lara