Source: Dev.to

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Introduction

While studying for CompTIA Network+, I couldn’t quite picture what the show vlan command displays. After learning what a VLAN is and what the command actually reports, everything clicked, so I decided to write it down.

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What is a VLAN?

VLAN = Virtual Local Area Network
It is a mechanism that divides a physical network into virtual segments.

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Why VLANs are needed

Without VLAN:
 → Everyone is in the same network
 → All data are shared across the entire organization regardless of department
 → Problems for security and performance

With VLAN:
 → Networks can be divided by department
 → Only sales staff can access data managed by the sales department
 → Security is improved

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Familiar example

Without VLAN:
 → Everyone works in one room
 → Every conversation can be heard by everyone

With VLAN:
 → Each department has its own private room
 → Conversations cannot be overheard by other departments

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Configuration diagram

【Without VLAN】

Switch
 ├─ PC (Sales department)
 ├─ PC (Development department)   ← Everyone in the same network
 └─ PC (Management department)

【With VLAN】

Switch
 ├─ VLAN 10 (Sales department)      ── PC・PC
 ├─ VLAN 20 (Development department) ── PC・PC   ← Networks divided per department
 └─ VLAN 30 (Management department) ── PC・PC

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What is the show vlan command?

It is the command to verify VLAN information on a Cisco switch.

Caution:
The show vlan command is exclusive to switches running Cisco IOS. Other vendors use different commands:

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Types of the command

1. show vlan

Shows all VLAN information in detail.

2. show vlan brief

Shows a simple overview of VLANs.

• The most used command.
• Frequently appears on exams.

3. show vlan id

Shows information for a specific VLAN.

Example

show vlan id 10

→ Displays only VLAN 10 information.

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Output example (show vlan brief)

VLAN Name                Status    Ports
---- -------------------- --------- -------------------------
1    default              active    Gi0/1, Gi0/2
10   Sales                active    Gi0/3, Gi0/4
20   Engineering          active    Gi0/5, Gi0/6
30   Management           active    Gi0/7
1002 fddi-default         act/unsup
1003 token-ring-default   act/unsup

Meaning of each column

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About the default VLAN (VLAN 1)

• All ports belong to VLAN 1 by default.
• It cannot be deleted.
• Its name cannot be changed from “default”.

Security caution

Using VLAN 1 in production is risky because a mis‑configuration can cause unintended traffic across the whole network.
Best practice: Move user ports to other VLANs.

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When to use the command

Troubleshooting

Verify that VLAN settings are correct.
Example: A PC cannot connect to the network → show vlan brief → discover that the port is not assigned to the intended VLAN.

Setting confirmation

After creating a new VLAN, run the command to confirm the configuration was applied correctly.

Adding new devices

Determine which VLAN a new device should join.
Example: Adding a new PC to the sales department → show vlan brief → note the sales VLAN ID → assign the port to that VLAN.

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Summary

VLAN:
 • Divides a physical network into virtual segments.
 • Provides security through network segmentation.

show vlan command (Cisco IOS):
 • Verifies VLAN information on a Cisco switch.

Main commands:
 • show vlan          → detailed information
 • show vlan brief    → overview (most frequently used)
 • show vlan id   → specific VLAN information

Typical use cases:
 • Troubleshooting
 • Configuration verification
 • Adding new devices to a VLAN

Cautions:
 • Command is Cisco‑specific.
 • Avoid using VLAN 1 for production traffic.

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VLAN Best Practices

• Default VLAN: Devices belong to VLAN 1 by default.
• Production environments: Do not use VLAN 1 in production.

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Conclusion

At first, I had no idea what the show vlan command was actually displaying. Once I understood that VLAN is a mechanism for dividing a network into virtual segments, the output — including the meaning of each column — fell into place naturally.