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

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

With VLAN：
　→ Be able to divide network depend on department
　→ Only sales staff can access data managed by the sales department
　→ Ensure security

Without VLAN：
　→ Everyone work in one room
　→ Every conversation can be heard to everyone

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

【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 ← Divide network into each department
　└── VLAN 30（Management department）── PC・PC

Caution：
The show vlan command is exclusive to switches running Cisco IOS 

Other vendors use other command：
　Juniper → show vlans
　HP/Aruba → show vlans
　Dell → show vlan

Show All VLAN information in detail

Show VLAN overview simply
　→ The most used command
　→ Frequently appears in the exam

Just show specific VLAN

例：
show vlan id 10
　→ Show only VLAN 10 information

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

VLAN：
　→ VLAN ID（ Identification number）
　→ Available range 1〜4094

Name：
　→ VLAN name

Status：
　→ active = Work normally
　→ act/unsup = Not supported

Ports：
　→ Ports assigned to the VLAN
　→ Gi = GigabitEthernet

VLAN 1 is a special case

・All ports belong to VLAN 1 as default
・Cannot be deleted
・Cannot be changed from name "default"

It is dangerous to use VLAN 1 itself
　↓
Reason：
Misconfiguration can cause unintended traffic across the network because all ports belong to VLAN 1 as default.

Measure：
It is common to move to other VLAN in production environment

Verifying whether VLAN setting are correctly configured .

Example：
PC cannot connect to network
　↓
Type "show vlan brief" and confirm the output
　↓
Discover problem that this port was not assigned to VLAN

After creating a new VLAN, run this command to verify the settings were applied correctly.

Example：
Confirm whether settings success correctly by type this command after new VLANN is created 

Setting after confirming "Which vlan we have to add to"

Example：
We want to add a new PC to the sales department network
　↓
Confirming VLAN ID for sales department by typing "show vlan brief" 
 ↓
Assigning the port to the VLAN

VLAN ：
To divide physical network into segment virtually
It can ensure security by network segmentation 

Show VLAN Command：
The command to confirm VLAN information on Cisco IOS Switch 

Main command：
show vlan → Show detailed information
show vlan brief → Show overview（ Most frequently used）
show vlan id XX → Show specific VLAN information

Use case：
・Troubleshooting
・Setting confirmation
・Adding new devices to VLAN

Caution：
・Command dedicated to Cisco IOS
・All port belong to VLAN 1 as default
・Don't use VLAN 1 in production environment