Well, this little thing right here is actually outdated. That's what I'm using as an example. It's a Packet Squirrel. It costs $30 commercially to buy one of them. I'm sure you could get a whole bunch of them at a bulk rate. It is sold online. I have no financial ties to it. I don't stand to benefit from it whatsoever.
I'm saying basically it is a network tap type of device. You just plug it in and it is connected to the network. It has software that runs on it that is configurable by you. You can set it to communicate with only one specific IP address, and set it to communicate in a way that it does not receive commands and only sends them.