Broadcast domain（廣播區域）是指能接收集合內任一設備發出的廣播幀所有設備的集合。
因為路由器不轉發廣播幀，廣播區域一般由路由器設定邊界。
A broadcast domain is the extent of a network where broadcast frames propagate. Basically, a broadcast domain covers an area where Layer 2 devices are located and terminates at the boundary of a Layer 3 device. In a switched network, the broadcast domain extends to all switch ports assigned to a common VLAN. This is because a switch forwards broadcasts out all available ports in a VLAN.
