|It might just be me (it's after 1.00 a.m. here), but your post is a little unclear.|
Are you trying to basically create a router? If so, I did it a few years back with 5 NIC's and a linux installation (not sure which flavor now, old age is creeping in).
Or, is this in order to test network performance? Have you try stopping the "routing and remote access" service (assuming Windows is being employed here)?
Maybe if I get some sleep and re-read your post in the morning, I'll understand what you need to accomplish more clearly. I apologise for any confusion I might have introduced to this thread.
Please let us know if you found someone's advice to be helpful.