A broadcast domain is the logical portion of a network where any device can directly transmit broadcast frames to other devices at the data link layer (OSI Layer 2). This simply means a broadcast message is sent to all devices connected to the same layer 2 network.
Below is an example of a broadcast domain on a Layer 2 switch. The computer with IP 192.168.100.100 sends a broadcast message and it is sent to every device in the same broadcast domain. Computer 192.168.100.101, 102, and any other device on this network will get the broadcast message.
If you have a hub or switch connected the broadcast message will also be forwarded to it and any connected device. In the below example, a hub is connected to the switch, the switch forwards the broadcast message and the hub forwards it to all of its ports.
To simplify the above, the 192.168.100.0/24 network and connected devices are part of a single broadcast domain.
Most enterprise switches allow you to break up a broadcast domain by creating virtual networks (VLANS). Routers can also break up a broadcast domain as they do not forward broadcast traffic.
In the example below, the computers in green are on VLAN 1, they are on their own broadcast domain. When a device from VLAN 1 sends a broadcast message it only goes to devices in the same VLAN. The computer in red is on a separate VLAN (its own broadcast domain) so it will not receive broadcast messages from any device in VLAN 1 and vise versa.
A broadcast message has the destination MAC address of FF:FF:FF:FF:FF:FF.
Here is a packet capture of a broadcast on my 192.168.100.0/24 network.
In the above image, the source device is sending a broadcast message out on the network. Because it is a broadcast message it will be sent to every device in the broadcast domain (the 192.168.100.0/24 network).
Here is one more diagram to illustrate broadcast domains. The computers are separated into VLAN 1 (192.168.100.0/24) and VLAN 2 (192.168.101.0/24), this creates two broadcast domains.
If a device in VLAN 1 sends a broadcast message it will go to all devices in the VLAN 1 network. If a device in VLAN 2 sends a broadcast message it will go to all devices in the VLAN 2 network.
Network Devices in a Broadcast Domain
Layer 1 and 2 network devices, such as hubs, bridges, and switches, create a single broadcast domain as they automatically forward broadcast messages.
Every network device connected to a hub is, by default, part of the same broadcast network. If one of these computers receives a broadcast message, the hub will automatically forward the message to all its interfaces, including other computers, switches, and routers.
A switch is a Layer 2 network device that separates a collision domain but extends a broadcast domain. This network device functions on the data link layer and provides full bandwidth to every transmission.
Unlike some hubs that indiscriminately broadcast traffic to every device with a network connection, a switch doesn’t transmit back to the device from where it received the broadcast. Adding a switch to a network allows for all three types of transmissions, including:
A Layer 3 network device, such as a router, can break broadcast domains into segments. When a router receives a broadcast message from a hub or switch, it drops this signal. In other words, you can add a router to your network to create multiple broadcast domains.
Implementing Broadcast Domains
Every network device in a broadcast domain that receives a broadcast must process the information.
For example, suppose a broadcast domain consists of 100 devices. If one computer transmits a broadcast, all the devices in the broadcast domain will receive a copy of the message, which they need to process.
Broadcasts within extensive domains can take up valuable resources and bandwidth. Network designers can add one or more routers to segment broadcast domains and make their networks more efficient.
However, the segmentation of broadcast domains with routers can be problematic.
For example, suppose a router connects two separate broadcast domains, and only one of them has a running dynamic host configuration protocol (DHCP) server that assigns IP addresses to systems. In this case, the router doesn’t transmit DHCP requests from one broadcast domain to another.
A possible solution is to use routers with DHCP-forwarding capabilities. These routers can separate broadcast domains to ensure optimal network efficiency while allowing DHCP request transmissions. On Cisco equipment, you can use the ip helper-address command to help forward DHCP requests if the DHCP server is in another network.
When designing a network, it is crucial to keep the characteristics of a broadcast domain in mind. It is recommended to break up a network into multiple broadcast domains. In large networks, it is not uncommon to see a very large broadcast domain using a /16 subnet (65,536 potential hosts).
If you have any questions about broadcast domains comment below.