Any network is composed of a number of hosts that are connected together. The ways the hosts are connected are referred to as the topology of the network. Normally if those hosts are connected through a layer 2 device such as switch or hub, the data sent by a PC are transmitted to all PCs that are connected to this one in the same broadcast domain.

From the above the concept of the broadcast domain has been arisen. When a PC send data that can be received b all other PCs n the network segment without layer 3 device, this segment is called broadcast domain. Thus the broadcast domain is the domain in which any network data can be propagated without the necessary use of routers.

To well understand the broadcast domain, consider a company that has two departments. The first is the management department while the second is the sales department. Each department has its own employees and each employee has a computer. Thus there will be two networks, one for the management department and one for the sales department.

When an employee sends a message to another employee across the network, this data will propagate across the network until the correct recipient find the message. Note that each network is based on switch and not routers. Thus the switch will forward the message to all other hosts on the network. This is considered like broadcast message in which the message is forwarded to all other hosts. But recall that if the switch has built a table that identifies the hosts on its ports with their MAC addresses, it will not forward to all ports in the broadcast domain

Now suppose that the two above networks of the two departments are connected together through switches, what will happen? If one employee sent a message to another employee in the same department (one broadcast domain), that message will be forwarded to all employees in his network and also to all employees in another department (another broadcast doain). This can break the security of information and also consumes bandwidth because the data are sent on links without the use of sending it to the other department.

So what is the solution in the above example? The designer must separate the entire network into two broadcast domains. Each department will be a broadcast domain. Thus when one employee in a department broadcasts a message to all other hosts in that department, the message will be received only by the employees on that department.Thus isolation has been done to prevent interaction between the two networks in different departments.

The way the network is splitted in different broadcast domains can be done by the use of routers. The router can have many interfaces to connect each network to. An interface called FA (fast Ethernet) followed by the number of the interface must be used to connect the switched network to the router. Each interface on the router corresponds to different broadcast domain. The interface can be connected to the interface of the switch or the hub.