Change IP address of http request

Hi All,
Our web site has code that makes its behaviour sensitive to the geographical location of the user. This is done using a third party tool that uses the IP address of the user. We need to test this part of our web site. So we need to simulate a request from various locations (IP addresses).
My question is this:
Is there a way to change the IP address that a request appears to have originated from?
NOTE: This must not be a code change in the site -our web site code must still get the IP from the ASP.Net Request object.
Environment:
IIS 6 on Windows server 2003 SP2
Cheers,
Geoff
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