[ppseafield]

Ethernet includes address and source identifiers so that multiple sources and destinations can be used on the same wire. As the sibling comment mentioned, hubs (not switches) simply broadcasted every frame to every connection, and the NICs at each end would have to determine if they were the intended destination. Some of them didn't even require power because they were essentially joining the wires together. Switches require power because they learn which destination the ports respond to and can route more efficiently based on that knowledge.