Collision Domain

A group of Ethernet or Fast Ethernet devices in a CSMA/CD LAN that are connected by repeaters and compete for access on the network. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions.
A collision domain is sometimes referred to as an Ethernet segment.


Broadcast Domain

Broadcasting sends a message to everyone on the local network (subnet).
Example for Broadcasting would be DHCP Request from a Client PC. The Client is asking for a IP Address.
But the client does not know, how to reach the DHCP Server. So the client sends a DHCP Discover paket
to EVERY PC in the local subnet (Broadcast). But only the DHCP Server will answer to the Request.


How to count them?

Broadcast Domain:

No matter how many hosts or devices are connected together.
If they are connected with a repeater, hub, switch or bridge, all these devices are in ONE Broadcast domain.
A Router is used to seperate Broadcast-Domains (we could also call them Subnets - or call them VLANs ).
So, if a router stands between all these devices, we have TWO broadcast domains.

Collision Domain:

Each connection from a single PC to a Layer 2 device (switch, bridge) is ONE Collision domain.
But be carefull with Bridges. To ONE of the Bridgeports, you can connect MORE than one pc, over an
(oldfashioned) 10Base2 or 10Base5 Bus-based Cable.
If 2 PCs are connected to a Bridge, like in the picture, this is ONE Collision Domain!
But if 5 PCs are connected with seperate cables to a switch, we have 5 Collision domains.
If this switch is connected to another switch or a router, we have one collision domain more.

If 5 Devices are connected to a Hub, this is ONE Collision Domain!
Each devices, which are conected to a Layer 1 device (repeater, hub) build all togeter ONE collision domain.

OK this gets really confusing when you have more than level 1 networking

Lets say you have 3 switches, each of which is connected to 2 PC's each. For some crazy reason these three switches are connected to a switch which is connected to a router, Now,the way i understand it, the router connection to the 1st level switch should be one broadcast domain and the the three connections to the three switches from this switch should be three additional broadcast domains so in total I should have four broadcast domains (1 router to switch + 3 switch to switch).

regarding collision domains, each switch connected to two PC's is one collision domains. I understand that. But when these three switches connect to the second switch then we have 6 collision domains and when this switch connects to the router we have 7 collision domains. The trainer tells me there are 9 collision domains. How on earth tis that happening?

Regards

Alright,
many have explained it, let me give you a practical scenario
There is a street and there are 8 houses 4 on each side.The street is two way street
Now each house has its own path way to the the main road. The car can travel both the ways on the main road. This is called a switch.
SInce there are 8 houses , in switching terms we have 8 ports and each post has its own collision domain, insense, unless and until you have data using the same path way, there is no way that the packet(car) collides with another packet(car) destined to another port(house).

Incase of a hub, there is only one collison domain, ie there are no 8 different path ways, there is only ONE single sided path way. So there are chances that cars from different houses might collide againt each other. The interesting part is,
if you have 2 8 port siwtches , the number of collison domains would be 16
and if you have 2 16 port hubs the number of collison domain would still be one 😁 as you would use one of the 8 ports of the hub to connect to another hub , thus making it into one single large collison domain.

Now , broadcast domain. Irrespective whether its switch or hub, the broadcast domains are still 1.
To limit the broadcast domain, or break the broadcast domain, we use few advanced technologies like implemetantion of VLANs. Otherwise as well, placing a simple Router between two switches would result in 2 BC domains.

FANTASTIC GUYS

i am not an IT student and have been admiring to do Cisco,have been reading the issue of collisions but now i get it.