What if you have native BACnet devices but some are on Ethernet and some are on ARCNET? Or suppose you have two separate ARCNETS, each with the maximum number of devices on them? The way to identify the separate networks is to give each a unique "network number." This, along with a protocol for deciding how and when messages should be passed from one network to the next, is the purpose of the "network layer protocol." The term "network layer" comes from the ISO Open Systems Interconnection Basic Reference Model, ISO 7498.