Day-18: Collision & Broadcast Domain.

By | May 22, 2022

Collisions will happen in an Ethernet Network when two devices simultaneously try to send data on the Shared Media since Shared Media is half-duplex and sending and receiving are not supported at the same time.

Collisions are a normal part of life in an Ethernet network when Ethernet operates in Half-duplex and under most circumstances should not be considered as a problem.

A Collision Domain is any network segment in which collisions can happen (usually in Ethernet networks). In other words, a Collision Domain consists of all the devices connected using a Shared Media (Bus Topology or using Ethernet Hubs) where a Collision can happen between any device at any time.

Collision Domain.

  • The collision domain defines the set of devices on which their frames could collide.
  • A collision occurs when two devices send a packet at the same time on the shared network segment.
  • The packets collide and both devices must send the packets again which reduces network efficiency.
  • Collisions are often in a hub environment because each port on a hub is in the same collision domain. All port of a hub resides in a single collision domain, For example, if we have 24 port hub it means we have only 1 collision domain.
  • By contrast, each port on a bridge, a switch or a router is in a separate collision domain. It means the switch breaks the collision domain, Each switch is considered a separate collision domain. For example, if we have a 24port switch it means we have 24 collision domains.

Collision Domain Identifying.

Example -1

In the above network, we have 6 collision domains.


In the above network, we have 8 collision domains.

Broadcast Domain.

Broadcast is a type of communication, where the sending device send a single copy of data and that copy of data will be delivered to every device in the network segment. Broadcast is a required type of communication and we cannot avoid Broadcasts, because many protocols (Example: ARP and DHCP) and applications are dependent on Broadcast to function.

All ports on a hub or a switch are by default in the same broadcast domain.

All ports on a router are in the different broadcast domains and routers don’t forward broadcasts from one broadcast domain to another.

Broadcast Domain Identifying.


In the above network, we have 3 Broadcast domains.


In the above network, we have 4 Broadcast domains.

In a Switch, By default, all ports are part of the same broadcast domain but we can make multiple broadcast domains in the switch by creating the VLAN.So VLAN is used for creating the broadcast domain in the switch.


As you can see above,2 ports are part of different VLANs so they will be considered as 2 broadcast domains,.


in the above diagram, We have 2 broadcast domains even if we have 2 switches . the VLAN basically controls the broadcast means it does not allow to enter one VLANs broadcast traffic into another vlan.

That’s how we make separation of Broadcast domain .

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