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Commonly confused · Routing & Switching

Collision Domain vs Broadcast Domain: What's the Difference?

A collision domain is a network segment where frames can collide because devices share the medium in half duplex. A broadcast domain is the set of devices that receive each other's Layer 2 broadcasts. Switches split collision domains; routers and VLANs split broadcast domains.

Collision Domain

Full definition
vs

Broadcast Domain

Full definition
Collision DomainBroadcast Domain
Concerned withShared media access (Layer 1/2)Layer 2 broadcast reach
Split bySwitches, bridges, routersRouters and VLANs (Layer 3 boundaries)
Not split byHubs, repeatersHubs, switches (without VLANs)
Problem it causesCollisions and retransmissionsBroadcast storms and excess flooding
Modern relevanceMostly eliminated by full-duplex switchingStill a key design concern
Counting ruleOne per switch or router portOne per router interface or VLAN
Remember it

Switches break collisions, routers break broadcasts.

Exam tip

Diagram questions ask you to count both. Every switch port is its own collision domain, a hub's ports all share one, and each VLAN or router interface is a separate broadcast domain.

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