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
hubrepeats every bit it receives out every other port — all connected devices are in one collision domain. - A
switchgives each port its own collision domain. Combined with full-duplex links, collisions are effectively eliminated on modern switched networks. - A
routeralso separates collision domains, one per interface.
Hubs are essentially extinct in production networks. Full-duplex switched Ethernet means collision domains are mostly a historical/exam 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 FFFF.FFFF.FFFF) sent by any one device in that domain.
- A
switchdoes not break up a broadcast domain — every port in the same VLAN 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.
Side by side
| Device | Breaks collision domain? | Breaks broadcast domain? |
|---|---|---|
| Hub | No | No |
| Switch | Yes — per port | No |
| Router / L3 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 requests, DHCP discovers, and other broadcast traffic gets flooded to hundreds of hosts that don't care — wasting bandwidth and CPU. This is the entire justification for VLANs: they let you keep broadcast domains small and purposeful without running separate physical switches for every segment.