2003
1. Click Start
2. Click Run
3. Type in cmd and hit ok (this opens a Command Prompt)
4. Type ipconfig /release and hit enter
5. Click Start, Control Panel, and open Network Connections
6. Find and Right click on the active Local Area Connection and choose Properties
7. Double-click on the Internet Protocol (TCP/IP)
8. Click on Use the following IP address
9. Enter a false IP like 123.123.123.123
10. Press Tab and the Subnet Mask section will populate with default numbers
11. Hit OK twice
12. Right click the active Local Area Connection again and choose Properties
13. Double-click on the Internet Protocol (TCP/IP)
14. Choose Obtain an IP address automatically
15. Hit OK twice
16. Go to 

For Vista (Windows 7 is very similar)
1. Click Start
2. Click All Programs expand the Accessories menu
3. In the Accessories menu, Right Click Command Prompt and choose Run as administrator
4. Type ipconfig /release and hit enter
5. Click Start, Control Panel, and open Network and Sharing Center. Depending on your view, you may have to click Network and Internet before you see the Network and Sharing Center icon
6. From the Tasks menu on the left, choose Manage Network Connections
7. Find and Right click on the active Local Area Connection and choose Properties (If you’re hit with a UAC prompt, choose Continue)
8. Double-click on Internet Protocol Version 4 (TCP/IPv4)
9. Click on Use the following IP address
10. Enter a false IP like 123.123.123.123
11. Press Tab and the Subnet Mask section will populate with default numbers
12. Hit OK twice
13. Right click the active Local Area Connection again and choose Properties
14. Double-click on Internet Protocol Version 4 (TCP/IPv4)
15. Choose Obtain an IP address automatically
16. Hit OK twice
17. Go to 

See if you have a new IP address

Some people have inquired about manually assigning their IP address. This IS possible, but you run a very high risk of your ISP banning you from connecting to the internet. To manually change your IP, follow the steps above for your Operating System. 

In step 9 or 10, depending on your O/S, enter an IP similar to the one displayed in the command prompt window. For example, if the IP displayed in the command prompt window is 75.1.2.3, change yours to 75.1.2.4. You’ll also need to manually enter the Subnet, Gateway and DNS Server IP addresses. If the new IP you give your computer doesn’t work, chances are someone else on the ISP network has already been assigned that IP. 

You’ll need to move on to the next one and keep trying until you find an open one. Keep in mind that some ISPs match up your MAC or modem data to the address that’s been assigned. If those 2 things don’t match up, you won’t be able to connect no matter what.

If your router gets the IP and not your computer, you’ll need to http in to your routers interface and manually assign the IP there. It’s pretty much the same method as assigning the IP to your computer. The method that varies is in how you reach your routers interface. You can get the instructions from your manufacturer’s website.