[kazinator]

MACS are random. Given a MAC and a connection to a LAN, you can easily answer the question, "is there a station with that MAC here". If it's not here, and you have a single gateway to another network, you can figure out that to talk to that MAC, you need to go over a gateway. And then things eventually go funny. We hit a network that talks to four others. It has no idea where to send the packet destined for that MAC. It could send it to all four (multicast). Then when a reply comes from one of them, remember that destination for next time. Remember for how long? Sending a packet to every destination will cause an exponential explosion of that packet throughout the network.

It works on small scales. We can stitch together a few LANs with ethernet switches. The switches initially forward everything to all ports, but learn where the MACs are so as to send frames only to ports where the destination MAC is known to be.

Ethernet switching won't scale to anywhere near the complexity of the Internet.