Broadcast Domain is the area that broadcasts are received, and broadcasts are restricted by routers.
In other words, if the router receives a broadcast it is dropped. If a layer 2 Ethernet switch with no VLAN configurations receives a broadcast request, it must be passed to all devices attached to it. This is what we call the broadcast domain, where the domain is the extent to which broadcasts are heard on the Internet or on the network.
Let's take for example with a standard company setup, we have a Carrier's router connecting to an ISP 's router (or customer router), connecting to an unmanaged switch, which is connected to a server and clients (PC's or IP Phones).
When client A broadcasts on the network that's an ARP request which is a broadcast and because it 's a broadcast everybody is obliged to listen whether you're a router or a server of a workstation, you are obliged to listen.
Because the router restricts the broadcast the extent to which the broadcast will be heard is restricted to that network and will travel no further than that router's interface.
In other words, the router to the Internet will not up-broadcast or will not relay that broadcast message and that is called a broadcast domain. So in order to break up broadcast domains a router can be used.
