What is a VLAN? - Definition, Function & Implementation

Instructor: Vignesh Sivabalan
VLAN is a logical partition of a local area network (LAN) into a different group(s). In this lesson, let us understand about VLAN, its functions and its underlying implementation.

What is a VLAN (Virtual Local Area Network)?

A Virtual Local Area Network (VLAN) refers to a logical grouping of different hosts in a similar broadcast domain. VLANs simplifies the task for the IT network administrators to divide & make groups in a network, based on their functional and security requirements without having to plug/unplug physical LAN cables or modify the existing IT network infrastructure.

Switch and Broadcast Domains

A switch is a physical networking device that connects multiple hosts on a Local Area Network (LAN) like our computers, laptops, tablets and more. The switch helps to route the data back and forth in-between hosts. A switch has many ports based on its models and manufacturers, and each port serves one host device on a LAN.

An interface is the logical connection and configuration on a switch port that helps in communication. For example, when a host system operated using one switch port through an Ethernet cable, this communication channel can be called as an interface.

VLANs get configured on a switch by placing few interfaces (like tagging) into a single broadcast domain. A single VLAN can spread across various switches but remain in the same subnet or the broadcast domain. VLANs are similar to physical LANs; the difference is that it allows grouping of the host devices, even if they on different switches.

Figure 1: Switch Ports and Interfaces

A broadcast domain in a switch refers to the domain that receives the broadcast messages. Broadcasting a message means to send it to all the connected host devices that are a part of that domain. By default, all the ports are grouped in a common broadcast domain in a switch or a hub. Hence, the switch floods (sends) the message received from one port to all the other connected ports; so all the hosts that are attached to these ports receive the same message. This process is called flooding.

Example of a broadcast domain in a switch:

Imagine that a switch has eight ports and eight different hosts are connected to these ports, if a broadcast message reaches the switch from one of these hosts- the switch forwards them to all other seven hosts as they are a part of the common broadcast domain, which comes configured from the factory.

In contrast, the ports on a router are not in a common broadcast domain; it treats each port separately.

Figure 2: Broadcast Domains in Switches

In the above diagram, there are two switches and two broadcast domains. Switch 1 has three hosts connected to it, and all the three hosts share the same broadcast domain. Similarly, all the three host systems of switch 2 belong to a common broadcast domain.

Functions of VLAN

  1. Virtual LANs provide mechanisms for making logical groups of end devices, though they are on different networks.
  2. VLANs increase the number of broadcast domains possible in a LAN by grouping various hosts with similar functions.
  3. Implementing VLANs reduces the security risks significantly, as the number of hosts connected on a broadcast domain decreases. This is done by configuring a separate VLAN for only the hosts with the sensitive information.
  4. VLAN offers flexible networking models which groups different users based on their departments (jobs/function), rather than just physical locations of that network.
  5. Changing users/hosts on a VLAN is easy. All it needs is a new port level configuration. If a user wants to move from one VLAN to another, a new port needs to be configured on the desired VLAN.

Basic Implementation of VLAN

Let us understand VLAN and its implementation with a real-world example.

  • An international company has two branches at two different physical locations (Branch A & B). Both the branches are connected to each other over a private LAN network.
  • Each branch has four individual departments namely Production, Research, Administration and the Human Resources (HR).
  • All these departments have their sub-systems and are spread across both the branches as follows:

Branch A is dedicated to the Production department, so it has 2 systems for Production, 1 system for Research, 1 for HR, and 1 for Admin.

And similarly,

Branch B is dedicated to the Research department, so it has 2 systems for Research work and 1 system for Production, 1 for HR and 1 for Admin.

Below is the data representation in tabular format for each branch and number of systems in them.

Branch Name Production Systems Research Systems Admin Systems HR Systems
Branch A 2 1 1 1
Branch B 1 2 1 1

Figure 3: Default Switch mode before VLAN Configuration

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