XML Package And IP Address

I have been developing an online payment system using classic ASP
which uses Msxml2.ServerXMLHTTP to submit an XML package to a 3rd party payment
system. The website the payment system is part of is one of many on a
Server 2000 / IIS5.0 installation. Each web site on my server has its
own IP address. The 3rd party payment system is hosted on another
server not under my control.

The issue I have is that when the XML package is submitted, the 3rd
party system is not registering the correct IP addresss for the
originating request from the website, it is registering the 'Core' or
first IP address on the server. The originating IP address is part of
their authentication model and so the XML submission is failing
because the wrong IP address is being passed through.