VLAN (Virtual LAN) — Practical + Interview Guide

VLAN (Virtual LAN) — Practical + Interview Guide

Definition (say this in interviews):

A VLAN (Virtual Local Area Network) is a logical segmentation of a Layer 2 network that separates devices into different broadcast domains on the same physical switch.

👉 In simple terms:

“Same switch, but different networks.”

🎯 What You Will Learn

• Network segmentation
• Broadcast isolation
• Basic switch configuration
• Real office network concept

🧪 Lab Setup (Build This in Cisco Packet Tracer)

🔹 Topology

• 1 Switch (2960)
• 4 PCs

🔹 VLAN Plan

⚙️ Step 1: Assign IP Addresses

• PC0 → 192.168.10.1
• PC1 → 192.168.10.2
• PC2 → 192.168.20.1
• PC3 → 192.168.20.2

Subnet mask: 255.255.255.0

⚙️ Step 2: Create VLANs on Switch

Go to Switch → CLI

enable
configure terminal

vlan 10
name HR

vlan 20
name IT

exit

⚙️ Step 3: Assign Ports to VLANs

Assume:

• PC0 → Fa0/1
• PC1 → Fa0/2
• PC2 → Fa0/3
• PC3 → Fa0/4

interface fa0/1
switchport mode access
switchport access vlan 10

interface fa0/2
switchport mode access
switchport access vlan 10

interface fa0/3
switchport mode access
switchport access vlan 20

interface fa0/4
switchport mode access
switchport access vlan 20

🧪 Step 4: Test Connectivity

✅ Same VLAN (Should Work)

From PC0:

ping 192.168.10.2

✔ Success

❌ Different VLAN (Should Fail)

From PC0:

ping 192.168.20.1

❌ Request timed out

🔍 What You Just Learned (IMPORTANT)

VLAN 10 and VLAN 20 are separate broadcast domains

Switch does NOT allow communication between VLANs

Even though:

• Same switch
• Same cable

👉 Still isolated

Simulation

⚠️ Common Mistakes

• ❌ Forgetting switchport mode access
• ❌ Wrong port mapping
• ❌ Same IP network for all PCs

🎯 Interview Questions (Must Know)

❓ What is VLAN?

👉 Logical segmentation of network at Layer 2.

❓ Why VLAN is used?

• Security
• Performance
• Broadcast control

❓ Can VLANs communicate?

👉 No, unless:

➡️ Inter-VLAN Routing is configured

❓ VLAN works on which layer?

👉 Layer 2 (Data Link)

🧠 Real-World Example

Office:

• HR department → VLAN 10
• IT department → VLAN 20

👉 They are isolated for:

• Security
• Traffic control

Disclaimer