Broadcast Domain

1. Broadcast Domain Definition

In Ethernet networks, In Ethernet networks, some messages are intended for all devices. These are called broadcasts.

But how far can a broadcast message travel?

That depends on the broadcast domain, the portion of the network where a broadcast frame can be received by other devices.

Let’s take a simple example:

Diagram illustrating the broadcast domain definition with PC1 sending a broadcast frame through switches SW1, SW2, and SW3, reaching PC2 and PC3.

PC1 sends an ARP request, which is a type of broadcast message used to find the MAC address of another host. The frame is sent with the destination MAC address FF:FF:FF:FF:FF:FF, meaning “to all devices on the local network.”

Since only switches are involved, the frame is forwarded out of all ports except the one it came in on.

As a result, the broadcast message reaches all connected devices, even those located behind other switches.
All of these devices are part of the same broadcast domain.

2. Broadcast Domain with Switches

Switches operate at Layer 2 and do not create boundaries for broadcast traffic.
This behavior is fundamental to the broadcast domain definition, as switches extend the broadcast domain across all connected ports.

Diagram showing a broadcast domain extended across multiple switches (SW1, SW2, SW3) connecting PC1, PC2, and PC3.

Each time a switch receives a broadcast frame, it forwards it out of all other interfaces.

This behavior continues from switch to switch, allowing the broadcast message to reach every device within the network.

3. Routers Break Broadcast Domains

Unlike switches, routers stop broadcast traffic by design.

Routers operate at Layer 3.

Whenever a broadcast frame reaches a router interface, it is not forwarded to other interfaces.

Network diagram showing how a router (R1) blocks a broadcast message sent by PC1, preventing it from reaching PC2 and PC3, thus creating separate broadcast domains.

PC1 sends a broadcast frame. R1 receives it but does not forward it to any other network.

As a result, PC2 and PC3 never receive the message.

Each router interface marks the boundary of a separate broadcast domain.

4. Multiple Broadcast Domains with a Router

Now let’s look at the full picture.

Every router interface creates a boundary.
Each connected segment becomes its own broadcast domain.

Network diagram showing three separate broadcast domains created by a router (R1), each connected to different switches and devices: PC1 on Broadcast Domain 1, PC2 on Domain 2, and PC3 on Domain 3.

Even though all devices are connected through switches, the router ensures that each network segment is isolated from the others when it comes to broadcast traffic.

5. Creating Separate Broadcast Domains

So far, we’ve seen that routers break broadcast domains by default.
But did you know that switches can do it too?

That’s where VLANs (Virtual LANs) come in.

A VLAN allows us to logically split a single physical switch into multiple virtual broadcast domains.

Switch SW1 with two VLANs. VLAN 1 and VLAN 2 create separate broadcast domains on the same switch.

Now that you understand the broadcast domain definition and how it applies to switches and routers, you’re ready to take the next step.

In the following lesson, we’ll dive into what VLANs are, how they work, and why they’re essential for network segmentation.