The Open Systems Interconnection (OSI) model is a conceptual framework that breaks down how data travels across a network into seven distinct layers. It was developed to standardise how different systems communicate, regardless of the hardware or vendor involved.
If you work in IT, networking or security, the OSI model is the shared language for diagnosing network issues and understanding where in the stack something is going wrong.
L7
L6
L5
L4
L3
L2
L1
The 7 Layers, Interactive Reference
📚 Click each layer to expandThe Application layer is the topmost layer and the one end users interact with directly. It provides network services to applications, it doesn't refer to the applications themselves, but to the protocols that let applications communicate over a network. This is where web browsing, email, file transfers, and DNS all live.
The Presentation layer is responsible for translating data between the application layer and the network. It handles data format conversion, character encoding (e.g. ASCII), compression, and crucially, encryption and decryption. TLS/SSL encryption operates here, making it central to securing data in transit.
The Session layer establishes, maintains, and terminates communication sessions between devices. A session is a persistent connection for the duration of a conversation, for example, when you browse a website, a session is created and kept alive while you interact with it. This layer also handles session checkpointing and recovery if a connection drops mid-transfer.
The Transport layer is responsible for reliable full data delivery between devices. It handles segmentation (breaking data into chunks), flow control, error checking, and reassembly. The two key protocols here are TCP (connection-oriented, guaranteed delivery) and UDP (connectionless, fast but no delivery guarantee). Port numbers also live at this layer, they identify which application should receive incoming data.
The Network layer handles logical addressing and routing, determining the best path for data to travel from source to destination across multiple networks. It assigns IP addresses (IPv4 and IPv6) and uses routers to forward packets. Unlike the Data Link layer which handles communication on a single network segment, the Network layer enables communication between different networks entirely.
The Data Link layer handles communication between devices on the same network segment. It divides data into frames and assigns each device a MAC (Media Access Control) address for identification within the local network. This layer has two sublayers: LLC (Logical Link Control) which manages flow and error control, and MAC (Media Access Control) which governs access to the transmission medium and manages which device can transmit at any given time.
The Physical layer is the foundation of the OSI model, it deals with the actual transmission of raw bits (1s and 0s) over a physical medium. This includes the cables, connectors, signal voltages, radio frequencies, and hardware specifications that make communication physically possible. Network topology (bus, star, ring, mesh) and transmission modes (simplex, half-duplex, full-duplex) are also defined at this layer.
How Data Actually Travels
📡 Data FlowWhen you send data, say, loading a webpage, it travels down the OSI stack on the sending device (being encapsulated at each layer), across the network, and then back up the stack on the receiving device (being de-encapsulated). Each layer adds its own header information on the way down, and strips it on the way up.
Why It Matters for Security
Security threats don't operate at a single layer, they target specific weaknesses at each level of the stack. Understanding which layer an attack operates at helps you choose the right defence:
References: BMC, OSI Model 7 Layers · Link11, Network Layer