2.1 Explain characteristics of routing technologies
📘CompTIA Network+ (N10-009)
Subinterfaces
A subinterface is a virtual interface created on a physical network interface. In simple terms, it’s like splitting one physical port into multiple logical connections so a single interface can handle multiple networks or VLANs.
Subinterfaces are commonly used in routers and Layer 3 switches to manage traffic from different networks efficiently.
1. Why Subinterfaces Are Used
- Multiple VLANs over a single physical interface:
Suppose you have one router interface connecting to a switch that carries multiple VLANs (Virtual Local Area Networks). Instead of using one physical port for each VLAN, you can create subinterfaces on the router for each VLAN. - Segmentation and routing:
Each subinterface can be assigned its own IP address, subnet, and VLAN tag, allowing devices in different VLANs to communicate through the router. - Efficient use of hardware:
Instead of having multiple physical ports for each network, subinterfaces let you maximize one port to carry many virtual networks.
2. How Subinterfaces Work
- Tagged VLANs (802.1Q):
Subinterfaces usually use 802.1Q tagging to differentiate between VLANs. This tag tells the router which VLAN the traffic belongs to. - IP addressing:
Each subinterface is given a separate IP address. This allows the router to route traffic between VLANs.
Example Configuration (Cisco-like syntax):
Router> enable
Router# configure terminal
Router(config)# interface gigabitEthernet 0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.10.1 255.255.255.0
Router(config-subif)# exit
Router(config)# interface gigabitEthernet 0/0.20
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.20.1 255.255.255.0
Router(config-subif)# exit
GigabitEthernet 0/0is the physical interface..10and.20are subinterfaces for VLAN 10 and VLAN 20.- Each subinterface has its own IP address for routing.
3. Key Exam Points About Subinterfaces
- Subinterfaces are logical, not physical.
You don’t need a separate cable for each subinterface. - Each subinterface can belong to a different VLAN.
This allows one physical interface to serve multiple VLANs. - IP addresses are assigned per subinterface.
Each virtual interface acts like a separate network connection. - 802.1Q tagging is required for VLAN traffic.
Without VLAN tags, the router cannot differentiate between VLANs. - Used in “router-on-a-stick” setups.
A common term on exams for a router connecting multiple VLANs via one physical interface.
4. Real IT Environment Example
- A company has 3 departments: HR, Sales, and IT. Each department is on its own VLAN (VLAN 10, 20, 30).
- The router has only one interface connected to the switch.
- Instead of using 3 physical ports on the router, the network admin creates 3 subinterfaces, one for each VLAN.
- Now the router can route traffic between departments while only using one physical interface.
5. Quick Exam-Friendly Summary
| Concept | Notes |
|---|---|
| Subinterface | Virtual interface created on a physical interface |
| Purpose | Route traffic between multiple VLANs on a single interface |
| IP Address | Each subinterface gets its own IP |
| VLAN Tagging | Uses 802.1Q tags to identify VLANs |
| Common Use Case | Router-on-a-stick configuration |
| Physical Ports | Only one needed for multiple VLANs |
✅ Remember: For the exam, focus on what subinterfaces are, why they are used, how VLAN tagging works, and the concept of router-on-a-stick. You may get questions asking which setup allows multiple VLANs over one interface—that’s subinterfaces in action.
