((Formatting updated.)) Thanks Sebastian I did some reading up on NATs, and I am trying to find someone in our IT department who knows how to set them up to implement my filter requirement. But we may have found a much easier and cheaper solution. We can't change the IP of the comms device, which is the same (10.2.10.1) on each network. We don't have control of the test PC software, but we can tell it the IP of the devices it must communicate with. We can change the setup of the PCs in each network which talk to the test PC, and we can change the hardware configuration of the test PC (add more network cards.) The solution is this: Set the IPs of the PCs which talk to the test PC to 10.2.10.11 (on NetA), 10.2.20.21 (on NetB), and 10.2.30.31 (on NetC), with netmasks all set to 255.255.0.0. Install 2 extra network cards into the test PC, and set the 3 IP addresses to 10.2.10.9, 10.2.20.9. and 10.2.30.9, all with netmasks = 255.255.255.0. The first network interface connects to NetA (with 10.2.10.11 etc.) The second network interface connects to NetB (with 10.2.20.21), and the third to NetC (with 10.2.30.31). That's it - as long as the software on the test PC is "well behaved", and is talking to network via Windows calls rather than talking directly to the card, Windows routes the messages sent by the test PC software to the three network cards according their destination IP address. I hope to find a spare network card this morning to test this idea - I'll post the results.