1.1 - The OSI model and networking concepts
Network+ N10-009 objective 1.1 covers the OSI model and core networking concepts. The seven layers, from Physical up to Application, give a shared map of where each function lives. IP addressing and routing between networks happen at Layer 3 (Network), while TCP and UDP port numbers operate at Layer 4 (Transport) - which is why a Layer 4 load balancer makes forwarding decisions using ports. In software-defined networking (SDN), the control plane primarily runs on a centralized controller, separated from the data plane that actually forwards packets. You should map a described function or device to its correct OSI layer and understand the control, data and management planes. Expect scenario questions that name an activity - routing, port-based decisions, addressing - and ask which OSI layer or SDN plane it belongs to.
Routing + IP addressing = Layer 3. TCP/UDP ports = Layer 4 (so L4 load balancer uses ports). SDN brains on a central controller = control plane; forwarding = data plane.
Practice questions
1. IP addressing and routing between networks happen at which OSI layer?
- Layer 2 (Data Link)
- Layer 3 (Network) (correct answer)
- Layer 4 (Transport)
- Layer 7 (Application)
Layer 3 (Network) handles logical addressing (IP) and routing between networks. Layer 2 uses MAC addresses within a LAN; Layer 4 handles TCP/UDP ports.
2. Where does the SDN control plane primarily run in a software-defined network?
- On a centralized controller (correct answer)
- On each individual switch
- Inside every host NIC
- In the cabling patch panel
SDN separates control from forwarding: a central controller programs the data plane on devices via southbound APIs. Traditionally control lived on each switch.
3. At which OSI layer do TCP and UDP port numbers operate?
- Layer 2
- Layer 3
- Layer 5
- Layer 4 (correct answer)
Ports are a Layer 4 (Transport) construct that identifies application endpoints. Layer 3 handles IP, Layer 2 handles MAC framing.
4. At which OSI layer does a Layer 4 load balancer make forwarding decisions?
- Transport (ports) (correct answer)
- Application (URLs)
- Network (IP only)
- Data link (MAC)
A Layer 4 load balancer distributes by IP and TCP/UDP port without reading application content. A Layer 7 balancer inspects URLs, headers and cookies.
5. At which OSI layer does a Layer 2 switch primarily operate?
- Physical
- Network
- Data link (correct answer)
- Transport
A Layer 2 switch forwards frames by MAC address at the data link layer. Routers work at Layer 3 (IP); hubs are Layer 1.
6. At which OSI layer does TLS session encryption logically operate?
- Layer 2 (Data Link)
- Layer 4 (Transport)
- Layer 6 (Presentation) (correct answer)
- Layer 3 (Network)
In the OSI model, encryption/formatting maps to the presentation layer (6); TLS runs above TCP and secures application data. IPsec by contrast operates at Layer 3.