Broadcast Domains

Broadcast domains are logical parts or divisions of a computer network. In a broadcast domain, all the devices can reach via broadcast at the data link layer (OSI  Layer 2). A Broadcast Domain consists of all the devices that will receive any broadcast packet; originating from any device within the network segment. All ports of the hub and switch in the same broadcast domain. Hub and Switches send broadcasts out all interfaces except the interface on which it received.   Routers do not transmit broadcasts. When a router receives a broadcast, it does not forward it out other interfaces. Therefore, each router interface connects a broadcast domain and broadcasts are only propagated within its specific domain. Routers can separate the boundaries of the broadcast domains. The figure below shows the broadcast domain with different network sizes.

broadcast domains

In the figure above there are three broadcast domains since all ports on a hub and a switch are in the same broadcast domain and all ports on a router are in a different broadcast domain.

In an Ethernet LAN; A devices use Address Resolution Protocol (ARP) which sends Layer 2 broadcasts to a known IPv4 address on the local network to discover the associated MAC address. The host also gets its IP address configuration using the Dynamic Host Configuration Protocol (DHCP) which sends broadcasts on the local network to find a DHCP server.

A large broadcast domain is a network that connects many hosts. A problem with a large broadcast domain is that these hosts can generate excessive broadcasts and negatively affect the network. In Figure 1, LAN 1 connects 400 users that could generate broadcast traffic resulting in:

The decrease in available Bandwidth: A large number of Broadcasts will decrease the total bandwidth of the network for normal traffic because the broadcast traffic is forwarded to all the devices in the domain.

The decrease in processing power of computers and network devices: Since the computers and network devices need to process all the broadcast packets received; a part of the CPU power spent on processing the broadcast packets. This will reduce the processing power of computers and network devices in a Broadcast domain.

Why We Need Subnets

Subnetting is the process which decreases network traffic and improves the network performance. It also makes management very easy. When you want to isolate some segments from some other administrator can easily do this. It also enables an administrator to apply security policies such as which subnets allowed or not allowed to communicate together. There are different ways of using subnets. Network administrators can set devices and services into subnets such as:

  • Different floors in a building
  • Different Organizations or the sections of any organizations
  • Types of Different Devices such as servers, printers, and hosts
  • Any other division that makes sense for the network.
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