One of the most interesting topics in CCNA curriculum deals with VLans known as Virtual LAN. It is layer-2 technology and can be implemented on switches to allow devices connected to remote switches to have as if they are connected to a single common switch. Additionally it can be used isolate certain ports of a switch by logically removing them from the broadcast domain of the switch.
A broadcast domain is basically a group of computers effected when one of the PC in the group broadcasts. For example in a typical LAN setting, where all the computers are connected with a hub or a switch, all the computers form a single broadcast domain.
To make it more clear suppose I have the following scenario where PCs -A, B and D belong to the accounts section and C and E belong to the admin section. Also pc G in building 2 belongs to the admin section as well. Computers from different section must not be able to communicate with each other and should not be present in the same LAN segment.
Ordinarily they all form a single LAN and any broadcasts will be seen by all the PCs. We need to implement a solution where A, B and D form a LAN and C, E, G form another LAN. PC F should be isolated from all of them. With VLAN we have the flexibility to segment our physical LAN to logical subnetworks and merge PCs from different physical networks together.
The next part will be about these commands and how to configure switches as well as routers to make them aware of VLANS. To make PCS residing in different VLANs communicate with each other we need a router.
If we want to partition our network why not use a router?
First of all routers are very expensive. Secondly they have a limited number of physical interfaces. For example if I want to create 20 subnetworks then I will need 20 physical interface, (such routers are very expensive) one for each subnetwork, while in the case of switches I need to create 20 VLANS, no additional hardware is required.
To summarize Vlans provide the following benefits:
* Reduced traffic since better network efficiency