So you are unplugging that connection and plugging it in to the Xbox via a crossover cable? Does the IP address stay the same when you plug in to the Xbox?
If so, just manually set the Xbox IP to 86.8.100.129, Subnet Mask 255.255.252.0. If not then set the xbox IP address to 1 more than the alternate PC IP address, and set the subnet mask to match.
Check that it works by pinging the xbox from the PC.