Collision Domains and Broadcast Domains

Collision Domains

- layer 1 of the OSI model

- a hub is an entire collision domain since it forwards every bit it receives from one interface on every other interfaces

- a bridge is a two interfaces device that creates 2 collision domains, since it forwards the traffic it receives from one interface only to the interface where the destination layer 2 device (based on his mac address) is connected to. A bridge is considered as an "intelligent hub" since it reads the destination mac address in order to forward the traffic only to the interface where it is connected

- a switch is a multi-interface hub, every interface on a switch is a collision domain. A 24 interfaces switch creates 24 collision domains (assuming every interface is connected to something, VLAN don't have any importance here since VLANs are a layer 2 concept, not layer 1 like collision domains)


Broadcast Domains

- layer 2 of the OSI model

- a switch creates an entire broadcast domain (provided that there's only one VLAN) since broadcasts are a layer 2 concept (mac address related)

- routers don't forward layer 2 broadcasts, hence they separate broadcast domains

With all this information, you can say that on your diagram, there are 2 broadcast domains (1 router that separates 2 LAN segments composed by one or many switches, with only 1 VLAN per segment).

There are 8 collision domains, one per pair of devices connected to each other (switch to router, switch to swich, switch to computer etc...) since we are talking about layer 1 concept (physical connection).


A hub creates a collision domain and a broadcast domain. Here is the theory behind it.
Since a hub is a layer 1 device and runs at half-duplex, there is always the chance (more like a guarantee) that there will be collisions. It doesn’t matter how many PC's are plugged into the hub they will all be affected by the same collision path and hence all part of the same collision domain which was created due to the fact they were plugged into the hub.

A hub also doesn’t understand anything about IP addresses, so when the signal is received by a port on the hub, it's job is to send that signal to every other device plugged into it (on all ports), this means that if a PC sent a broadcast message, all devices would receive it.

The big thing to understand here, is that a hub does create a single broadcast and collision domain, but does not make more as we will see with the switch.

A switch is a layer 2 device which means it has some intelligence in the form of learning Mac addresses. This means that when a PC with mac address 'A' wants to send data to a PC with mac address 'B' the switch will only send the data to the corresponding port. This means that multiple pc's could have conversations at the same time. Switches also make use of buffers which allow data to be held back and sent when the line is free and hence prevents any collisions from occurring. Each port on a switch will be its own collision domain (assuming the port has been configured as half-duplex), in a case where each port has been configured as full-duplex no collisions should ever occur. This means that a switch creates collision domains (1 per port, in theory).

A switch however does not understand IP addresses (assuming this is a standard switch, and not a layer 3 switch or multilayer switch) and therefore doesn’t understand broadcast messages. This means that when a broadcast is sent from a host attached to the switch it will send the traffic to all ports. Since all machines on the switch will receive the broadcast message, they are said to be in the same broadcast domain.