QUICK TIP - Broadcast Domain – Simple Definition and 2 Easy Detailed Explanations

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The term broadcast domain (BD) describe a group of devices on a specific network segment that can reach each other with Ethernet broadcasts. We can not forward a device in one BD sending Broadcasts to devices in another broadcast domain. This improves the performance of the network because not all devices on a network will receive and process broadcasts.

Routers separate a LAN into multiple BD (every port on a router is in a different broadcast domain). Switches (by default) flood Ethernet broadcast frames out all ports, just like bridges and hubs. All ports on these devices are in the same BD.

To better understand the concept of broadcast domains, consider the following example:

In the picture below we have a network of six computers, two hubs, a bridge, a switch, and a router. The broadcast domains are marked in red lines. Remember, all devices connected to a hub, a bridge, and a switch are in the same broadcast domain.

Only routers separate the LAN into multiple broadcast domains. That is why we have four broadcast domains in the network pictured below.

Broadcast Domain
4 Broadcast Domains in red line

Base on the overview and example, we will give you a quick definition of broadcast domain and a simple example to easily understand and memorise.

What is a Broadcast Domain?

The Quick Definition: A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer.

Let’s say you have several computers connected to a Layer 2 switch. It is the hardware to learn and store the MAC address (also known as the physical address) of a device’s network cards.

By storing those hexadecimal numbers, the switch can make intelligent decisions about how to route packets to and from a specific device.  We can also configure Layer 2 switches with VLANs to service multiple networks. Since the switch has learned about the connected devices, it also knows (from the switch configuration) what network we connect to those devices.

What exactly a broadcast domain does

From our definition of a broadcast domain, we can also state that a broadcast describes a process that forwards packets to all machines on the network. Only devices communicating within a given network will receive the broadcast traffic.

Also, broadcasts do not cross network boundaries, i.e. broadcasts are not routable from one network to another. If they did, you can imagine the amount of traffic that would be generated across networks and/or the internet!

How MACs figure into the equation

We mentioned the MAC address earlier. This is a unique address, and we will add to all network interfaces. It is a hexadecimal value like ac:de:48:00:11:22. A Layer 2 switch stores these values and knows exactly what port on the switch connected to the MAC Address.

When the device (with MAC Address: ac:de:48:00:11:22) receive a packet, the switch immediately forwards the packet out of the connected port and straight to the device. No other devices on the network have to listen to or process that packet. This helps to reduce the amount of ‘noise’ and overall traffic on a given network.

A broadcast packet has a specific hexadecimal value for the destination MAC address: ff.ff.ff.ff.ff.ff.

When a switch receives this value, it knows to forward the traffic to all devices on the network where the broadcast originated. If you have 24 devices connected to a given network or VLAN on your Layer 2 switch, all 24 devices will receive and process all broadcast packets on that VLAN.

Broadcast domains in action

We show you a common example of a broadcast (You can review DORA process for details). Firstly, a device using DHCP adds an IP address, dynamically. Next, when the device requests an IP address, it sends a broadcast to look for a DHCP server. If DHCP server is available, the server will send the next available IP address directly to the device sending the request.

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