📊 The OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework that describes how data travels across a network in 7 layers.

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📚 OSI Layers Explained

Q1What is the OSI model?Intermediate▼

The OSI model is a 7-layer conceptual framework that standardises how different network systems communicate. Each layer has a specific role and communicates with the layers directly above and below it.

The 7 layers (top to bottom):

Layer 7 — Application: User-facing protocols (HTTP, FTP, DNS, SMTP)
Layer 6 — Presentation: Data formatting, encryption/decryption (TLS, JPEG)
Layer 5 — Session: Manages sessions between applications
Layer 4 — Transport: End-to-end communication, reliability (TCP, UDP)
Layer 3 — Network: Routing and IP addressing (IP, routers)
Layer 2 — Data Link: MAC addresses, switches, frames (Ethernet)
Layer 1 — Physical: Raw bits over cables/wireless (cables, hubs, NICs)

💡 Memory trick: "All People Seem To Need Data Processing" (Application, Presentation, Session, Transport, Network, Data Link, Physical)

Q2What happens at Layer 3 of the OSI model?Intermediate▼

Layer 3 — the Network layer — handles logical addressing (IP addresses) and routing. It determines the best path for data to travel from source to destination across multiple networks.

Key components at Layer 3:

Routers — operate at Layer 3, route packets between networks using IP addresses
IP protocol — assigns logical addresses to devices
ICMP — used by ping and traceroute to diagnose networks

⚠️ Layer 3 deals with logical addresses (IP). Layer 2 deals with physical addresses (MAC). A switch is Layer 2; a router is Layer 3.