Perhaps if you describe exactly what you are trying to accomplish, another way may be found. Changing your IP address (falsely) is generally seen as a very anti-social [1] thing to do. If you need some way to distinguish requests, almost any other variable (e.g. User-agent, query_info) would be a better choice to change.

[1] If you are browsing your own sites, of course, you can be as anti-social as you like :)