Networking Fundamentals · lesson 1 of 17 · 15 min
The OSI model The OSI (Open Systems Interconnection) model is a conceptual framework that divides network communication into seven layers. Each layer has a specific job and communicates with the layers directly above and below it. Layer 7 — Application: HTTP, DNS, SMTP — what the user interacts with. Layer 6 — Presentation: encryption, compression, encoding (TLS lives here conceptually). Layer 5 — Session: establishes, manages, and terminates sessions between applications. Layer 4 — Transport: TCP and UDP — reliable or unreliable delivery, port numbers. Layer 3 — Network: IP — logical addressing and routing between networks. Layer 2 — Data Link: Ethernet, Wi-Fi — MAC addresses, frames, switching within a network. Layer 1 — Physical: cables, radio waves, electrical signals — the actual bits on the wire. Mnemonic (top to bottom): “All People Seem To Need Data Processing” — Application, Presentation, Session, Transport, Network, Data Link, Physical.
OSI model quiz
A firewall that inspects HTTP request headers and blocks based on URL patterns operates at which OSI layer?
- Layer 3 — Network
- Layer 4 — Transport
- Layer 7 — Application
- Layer 2 — Data Link
Answer: Layer 7 — Application
HTTP is an application-layer protocol. A firewall that understands HTTP content is a Layer 7 (application-layer) firewall.
MAC addresses operate at which OSI layer?
- Layer 1 — Physical
- Layer 2 — Data Link
- Layer 3 — Network
- Layer 4 — Transport
Answer: Layer 2 — Data Link
MAC addresses are used by Ethernet and Wi-Fi to identify devices on the same local network segment — that is the Data Link layer.
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