OSI Model | Notion

OSI (Open Systems Interconnection) model is a seven‑layer framework that standardizes how network devices communicate. Learning these layers helps you understand where different protocols and hardware operate, troubleshoot effectively, and grasp terms like “Layer 3 switch” or “Layer 7 firewall.”

A popular mnemonic to remember them (bottom→top) is:

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(Physical, Data Link, Network, Transport, Session, Presentation, Application)

Layer	Name	Primary Function	Key Protocols & Devices
7	Application	Interface for end‑user applications to request network services	HTTP/HTTPS, FTP, SMTP, DNS; Browsers, Email clients
6	Presentation	Data translation, encryption, compression	SSL/TLS, MIME, ASCII/Unicode, JPEG, PNG
5	Session	Establishes, manages, and terminates sessions	RPC, NetBIOS, PPTP
4	Transport	Reliable (or connectionless) end‑to‑end data delivery, flow control, segmentation	TCP, UDP; Firewalls, Load‑balancers
3	Network	Logical addressing and routing between networks	IPv4/IPv6, ICMP; Routers, Layer 3 switches
2	Data Link	Framing, physical addressing (MAC), error detection	Ethernet (802.3), Wi‑Fi (802.11); Switches, Bridges
1	Physical	Transmits raw bits over physical medium	Cables (Cat5/6), Fiber, Wi‑Fi radios; Hubs, Repeaters

Physical (Layer 1)

Defines the electrical/optical/radio signals that carry raw bits (0s & 1s) over cables or wireless.
Hardware examples: Ethernet cables (Cat5/Cat6), fiber optics, Wi‑Fi radios (2.4 GHz, 5 GHz, 6 GHz), hubs, repeaters.
Key function: bit‑level transmission, media specifications, connector types, voltage levels.

Data Link (Layer 2)

Packages raw bits into frames, adds physical addressing (MAC).
Ensures reliable delivery on the same network segment.
Devices & protocols: switches, bridges, Ethernet (802.3), Wi‑Fi (802.11), MAC addresses.
Frame structure: destination MAC ⇢ source MAC ⇢ EtherType/length ⇢ payload ⇢ CRC.

Network (Layer 3)

Routes packets between different networks using logical addressing (IP).

Physical (Layer 1)

Data Link (Layer 2)

Network (Layer 3)

Physical (Layer 1)

Data Link (Layer 2)

Network (Layer 3)