2.4 Explain common network configuration concept
📘CompTIA A+ Core 1 (220-1201)
1. What Is a VLAN?
A VLAN (Virtual Local Area Network) is a logical network created inside a physical network.
- A physical LAN is based on cables, switches, and devices
- A VLAN groups devices by configuration, not by physical location
Devices in the same VLAN:
- Can communicate directly with each other
- Are logically separated from devices in other VLANs
Even if devices are connected to the same switch, they cannot communicate unless they are in the same VLAN.
2. Why VLANs Are Used (Exam Focus)
VLANs are used to:
1. Improve Security
- Devices in different VLANs cannot see each other
- Sensitive systems (e.g., servers) can be isolated
2. Reduce Network Traffic
- Broadcast traffic stays inside the VLAN
- Improves performance and reduces congestion
3. Better Network Organization
- Devices are grouped by function, not location
- Easier to manage large networks
4. Simplify Network Management
- Moves and changes are done by configuration
- No need to re-cable devices
3. VLANs vs Physical LANs (Important for Exam)
| Feature | Physical LAN | VLAN |
|---|---|---|
| Based on | Physical cabling | Logical configuration |
| Flexibility | Low | High |
| Security | Limited | Stronger |
| Broadcast control | Poor | Controlled |
| Exam relevance | Basic | Very important |
4. How VLANs Work
VLANs work at Layer 2 (Data Link layer) of the OSI model.
- Switch ports are assigned to VLANs
- Each VLAN acts as a separate network
- Devices in different VLANs:
- Cannot communicate without routing
5. VLAN ID (Very Important for Exam)
Each VLAN has a VLAN ID (VID).
- VLAN IDs range from 1 to 4094
- Used to identify VLAN traffic
Common VLAN IDs:
- VLAN 1 → Default VLAN (all ports initially)
- VLAN 10, 20, 30 → Commonly used for segmentation
6. Access Ports vs Trunk Ports (High Exam Value)
Access Port
- Carries traffic for one VLAN only
- Used for end devices:
- PCs
- Printers
- IP phones
Exam keyword: Single VLAN
Trunk Port
- Carries traffic for multiple VLANs
- Used between:
- Switch to switch
- Switch to router
- Switch to firewall
Exam keyword: Multiple VLANs
7. VLAN Tagging (802.1Q)
When traffic passes through a trunk port, it must be tagged.
IEEE 802.1Q
- Industry standard for VLAN tagging
- Adds a VLAN tag to Ethernet frames
- Identifies which VLAN the traffic belongs to
Access ports → Untagged traffic
Trunk ports → Tagged traffic
8. Native VLAN (Exam Concept)
- The native VLAN carries untagged traffic on a trunk port
- By default, the native VLAN is VLAN 1
- Best practice:
- Change native VLAN for security
9. VLAN and Broadcast Domains
Each VLAN is a separate broadcast domain.
- Broadcasts stay inside the VLAN
- Broadcasts do NOT cross VLAN boundaries
This:
- Improves performance
- Limits unnecessary traffic
10. VLAN Communication (Inter-VLAN Routing)
Devices in different VLANs cannot communicate directly.
To allow communication, you need:
- A router
- Or a Layer 3 switch
This process is called Inter-VLAN Routing.
11. VLANs in an IT Environment (No Analogies)
In a typical IT setup:
- Workstations are placed in one VLAN
- Servers are placed in another VLAN
- Guest systems are placed in a separate VLAN
- Network devices are placed in a management VLAN
This improves:
- Security
- Performance
- Network control
12. VLANs and IP Addressing
Each VLAN usually has:
- Its own IP subnet
- Its own default gateway
Example:
- VLAN 10 → 192.168.10.0/24
- VLAN 20 → 192.168.20.0/24
13. Common VLAN Terms You MUST Know for the Exam
- VLAN
- VLAN ID
- Access port
- Trunk port
- 802.1Q
- Native VLAN
- Broadcast domain
- Inter-VLAN routing
14. Common Exam Scenarios
You may see questions like:
- Why devices cannot communicate on the same switch
- How to separate departments securely
- How to reduce broadcast traffic
- Which port type supports multiple VLANs
- What protocol is used for VLAN tagging
15. Key Exam Takeaways (Memorize)
✔ VLANs logically separate networks
✔ VLANs improve security and performance
✔ VLANs operate at Layer 2
✔ Trunk ports carry multiple VLANs
✔ 802.1Q is the VLAN tagging standard
✔ Inter-VLAN routing requires a router or Layer 3 switch
