Windows

- Computer connected directly to the modem
Get to a command prompt. (START, run, cmd).

Type "ipconfig /release" (without the quotes, on the command line by itself).
Type "ipconfig /renew" (without the quotes, on the command line by itself).
Windows (second option)

- Computer connected directly to the modem
Get to a command prompt. (START, run, cmd).
Type "ipconfig /release" (without the quotes).
Shut down computer.
Turn off computer.
Turn off all ethernet hubs/switches.
Turn off cable/DSL modem.
Leave off overnight.
Turn everything back on.
Network with Router

Log into the router's admin console. (Often [ Register or Signin to view external links. ] )
Release the IP address. (Method varies by router manufacturer)
Turn off router, ethernet hubs/switches, and the cable/DSL modem.
Leave off overnight.
Turn everything back on.
If you are using a cable/DSL modem and a router, you may wish to connect your computer directly to the cable/DSL modem. Please note that this could significantly impact your system security. This allows your ISP's DHCP to issue you a new (hopefully changed) IP address based of the (hardware) MAC address of your computer's ethernet card.

If all the above has not worked to change your IP address and you have a router, check and see if there is a "Clone MAC Address" option. Using it should change your IP address; however, you'll only be able to do it once (in most cases).

These will not work in all cases. If all else fails contact your internet service provider (ISP) and ask them if they are able to change your IP address or how long your connection needs to be off for your IP address to change.

If you trying to change your IP address because you are just trying to access web based forums you may wish to attempt to configure your internet browser to use a proxy server.