WDS on core network need's talk to a different network.

I have set up our companies WDS server, which runs perfect when connecting to computers on the same network.

But we have a Student domain which has around 50 PC's connected to it which runs on a completely different network. We need the main WDS server to talk to the other PC's. We have added the appropriate rules on the firewall, and I can \\server\c$ to the main WDS server but when booting to PXE it doesn't seem to want to find the correct boot file.

I can't seem to work this one out.