Collision Domain vs Broadcast Domain

Collision domain

A collision domain is a segment of network where two devices transmitting at the same time can collide: their signals interfere on the shared medium and both transmissions are corrupted. This only matters on shared, half-duplex media.

A hub repeats every bit it receives out every other port, so all connected devices are in one collision domain.
A switch gives each port its own collision domain. Combined with full-duplex links, collisions are effectively eliminated on modern switched networks.
A router also separates collision domains, one per interface.

WHY THIS RARELY COMES UP ANYMORE

Hubs are essentially extinct in production networks. Full-duplex switched Ethernet means collision domains are mostly a historical concept today, but it's still the correct mental model for why switches replaced hubs.

Broadcast domain

A broadcast domain is the set of devices that receive a Layer 2 broadcast frame (destination MAC, or Media Access Control, address FFFF.FFFF.FFFF) sent by any one device in that domain.

A switch does not break up a broadcast domain, so every port in the same VLAN (Virtual Local Area Network) floods and receives broadcasts together.
A router (or a Layer 3 switch doing inter-VLAN routing) does break up broadcast domains: each subnet/VLAN is its own broadcast domain.
VLANs exist specifically to create multiple broadcast domains on a single physical switch, without needing separate hardware per segment.

Left: every device shares one collision domain through the hub. Right: two switches each form their own broadcast domain. A broadcast from a host behind SW-A never reaches SW-B unless R1 explicitly routes it, which routers don't do for broadcasts.

Side by side

Device	Breaks collision domain?	Breaks broadcast domain?
Hub	No	No
Switch	Yes, per port	No
Router / Layer 3 switch	Yes	Yes

Why it matters in practice

Broadcast domain size is the real design constraint in modern networks. Too large a broadcast domain means ARP (Address Resolution Protocol) requests, DHCP (Dynamic Host Configuration Protocol) discovers, and other broadcast traffic gets flooded to hundreds of hosts that don't care, wasting bandwidth and CPU (Central Processing Unit) cycles. This is the entire justification for VLANs: they let you keep broadcast domains small and purposeful without running separate physical switches for every segment.