IP addresses are dynamicly assigned, meaning you get a new one everytime you log onto the internet.
You can ask your ISP to assign you a static IP address, for an extra charge, but you only need that if you're planning on doing funky stuff with networking, such as VPN.
The easy way to change your IP:
-turn off your cable/dsl modem for 15-30 secs.
- if your PC is the only one attached to the modem: go to start-->run, type cmd, hit enter, in the black window type this command: ipconfig /release
- if you're on a home network: open your browser, go to your router interface by typing in the address line: 192.168.1.1 or 192.168.0.1(depending on the brand of router), enter your admin username and pw, go to "router Status" or "Connection Status", hit the release/renew button.
This last one is a bit complex, but handy if you're sharing your connection with your landlord and can't physically get to the modem.
Hope this helps