Hub

A hub is a physical layer device that connects several hosts in a star topology. It has no understanding of addresses: whatever arrives on one port is repeated out of every other port. The attached hosts therefore share one collision domain and one broadcast domain, taking turns on the medium using CSMA/CD, which limits throughput as more hosts join. Hubs have largely been replaced by switches but remain the reference device for layer 1 of the OSI model.

The security consequence is simple. Because every frame reaches every port, any host on a hub can capture the others’ traffic through packet sniffing, needing only a network card in promiscuous mode. A switch at layer 2 learns which MAC address sits on each port and forwards unicast frames only where they need to go, as a bridge does between segments. A router at layer 3 goes further and separates broadcast domains. MAC flooding aims to fill a switch’s address table so that it floods frames like a hub, restoring the eavesdropping a hub gives for free.

Exam relevance: questions in this area tend to match devices to OSI layers and to their collision and broadcast domains. One host seeing every other host’s traffic is likely to point to a hub, or to a switch forced to behave like one.