I would need more information, but will try to make a guess (assuming you can work with a Linux distribution):
If ("multiple IP access" doesn't mean different IPs) { If (your application doesn't need interaction, but only an http request) { I would try several wget in parallel launched from a shell script } else // == your application needs interaction { Can you program a light client (let's say python, java...) to simulate one client and launch several simultaneously? } } else // == you need different IPs { If (the IPs need to be public) { I would try to work on wget/light client accesing different proxy servers on the Internet. } else // == they can be private { I would first try to create about 500 subinterfaces (eth0:n). This means 2 /24, and some work on address space on your private site. Once you have the subinterfaces, I would try the wget/light client over different subinterfaces. } }