a broadcast domain

A broadcast domain is the set of devices that all receive a frame when one of them shouts to everybody at once. Picture a single open-plan office: if someone stands up and yells an announcement, everyone in that room hears it, but people in other rooms or other buildings do not. The group that hears each broadcast is one broadcast domain.

In Ethernet, a broadcast frame has the special destination address ff:ff:ff:ff:ff:ff, meaning 'everyone on this LAN'. Switches and bridges forward such a frame out every port (except the one it came in on), so an entire switched network is one broadcast domain by default. Broadcasts are essential — ARP uses one to ask 'who has this IP address?' and DHCP uses one to find a server when a host first joins — but too many machines in one broadcast domain means every broadcast interrupts everyone, which wastes bandwidth and CPU. A router stops broadcasts at its boundary: it does not forward a layer-2 broadcast to another network, so a router separates broadcast domains. A VLAN does the same logically, carving one physical switch into several independent broadcast domains.

The key contrast: a switch breaks up collision domains but not broadcast domains. To split a big flat network into smaller, quieter pieces — for performance and for security isolation — you use routers or VLANs, not more switches.