Collision and Broadcast Domain Explained

1. Introduction

Understanding collision and broadcast domains is critical for designing efficient networks. Knowing how a collision and broadcast domain affects communication is essential for optimizing performance and avoiding congestion.

2. Collision Domain

Definition

A collision domain is a part of a network where data packets can collide if multiple devices attempt to transmit simultaneously. Collisions slow down network performance as devices must retransmit data.

Example with a Hub

  • In a network using a hub, all devices share the same collision domain. If two devices transmit at the same time, a collision occurs.

Hubs lack intelligence—they send incoming data to all ports without distinguishing the destination.

Illustration of collision and broadcast domain behavior in a hub-based network.

How a Switch Reduces Collisions

A switch isolates each port into its own collision domain.

This means:

  • Each device connected to the switch communicates independently.
  • No collisions occur between devices.

Switches eliminate collisions, making them far superior to hubs for modern networks.

Illustration of collision and broadcast domain separation with a switch.

3. Broadcast Domain

Definition

A broadcast domain is a network segment where all devices receive every broadcast frame sent by any device within that segment. Broadcast communication is one-to-all, meaning every device in the domain gets a copy of the broadcast.

Broadcast frames have a destination MAC address of FF-FF-FF-FF-FF-FF. When a broadcast frame is received by a switch, it floods the frame to all ports except the one it was received on.

Single Broadcast Domain: Hub Example

In a network with a hub, all connected devices belong to the same broadcast domain. When one device sends a broadcast frame, all other devices in the segment receive it. The hub does not distinguish between devices; it simply forwards the frame to all ports.

Example of collision and broadcast domain in a hub-based network.

The diagram below shows a hub with three connected devices, all part of one broadcast domain.

Single Broadcast Domain: Switch Example

When a switch is introduced, the logic remains the same. A broadcast frame sent by one device is flooded to all other devices connected to the switch. All ports on the switch belong to the same broadcast domain by default.

Visualization of collision and broadcast domain behavior in a switch-based network.

The diagram below demonstrates that even with a switch, all devices still share a single broadcast domain.

How a Router Solves This

A router creates separate broadcast domains by stopping broadcast frames. Unlike switches, routers do not forward broadcasts. Instead, they decapsulate Ethernet frames and work with Layer 3 (IP) information to decide where to forward traffic.

Example with a Router

In the following network:

Diagram showing how a router separates collision and broadcast domains.
  • The switch still maintains a single broadcast domain for PC1, PC2, and PC3.
  • The router divides the network into multiple broadcast domains:
    • Domain 1: PC1, PC2, and PC3 connected to the switch.
    • Domain 2: PC4 connected to the router.
    • Domain 3: PC5 connected to the router.

Broadcast traffic from one domain does not reach the other domains, as the router blocks it.

Power Over Ethernet (PoE)