What is the difference between a collision domain and a broadcast domain? Count them in an example.
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A collision domain is the set of devices whose transmissions can collide on a shared half-duplex medium. Hubs extend it; every switch port and router port is its own. A broadcast domain is the set of devices that receive a layer 2 broadcast. Switches extend it within a VLAN; routers and VLAN boundaries end it. With full duplex on every port, collisions cannot happen at all.
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Worked example
R1 has two LAN interfaces. Gi0/0 connects to switch SW1 with four PCs. Gi0/1 connects to a hub with three PCs. All on one VLAN each.
- Broadcast domains: 2. One per router interface. R1 does not forward broadcasts.
- Collision domains: 6. SW1 has five ports in use (four PCs and the uplink to R1), each its own collision domain. The hub, its three PCs and R1 Gi0/1 share one.
Rules of thumb
| Device | Collision domains | Broadcast domains |
|---|---|---|
| Hub | 1 for all ports | 1 |
| Switch, one VLAN | 1 per port | 1 |
| Switch, N VLANs | 1 per port | N |
| Router | 1 per interface | 1 per interface |
Why it matters now
Collision domains are history on a full-duplex switched network. Broadcast domains still matter: every host in one must process every ARP and broadcast, and a loop in one becomes a broadcast storm. That is why we keep VLANs to a few hundred hosts.
They will ask next
Does a VLAN create a collision domain or a broadcast domain?
A broadcast domain. Collision domains are per port regardless of VLAN.
Is a Wi-Fi cell a collision domain?
In effect, yes. Stations share one channel and contend for it. Wi-Fi avoids collisions with CSMA/CA because a radio cannot detect a collision while it transmits.
Traps
- Saying switches break up broadcast domains. They break up collision domains.