因为路由器不转发广播帧，广播区域一般由路由器设定边界。

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.

一个广播帧能广播到的整个的网路区域成为一个广播域,基本上,当第二层设备位于并终止于第三层设备和它的边界之间,那整个这个区域就是广播域覆盖的範围.在一个交换网路中,广播域可以扩展到所有被指派到一个公共的虚拟网路的所有交换连线埠中.这是因为在同一个虚拟网路中,交换机能转发通信到所有可以用到的连线埠.