Networking is broken into layers where each layer handles a single specific job and relies on the layer directly below it. This modular separation allows each layer to be designed and modified independently without disrupting the entire system.

The classic teaching framework is the Open Systems Interconnection (OSI) model, which organizes network communication into seven layers from the physical medium at the bottom to user-facing software at the top:

1. **Physical:** Transmits raw signals over physical media, including actual cables, radio waves, and electrical voltages.
2. **Data Link:** Moves data between two directly connected devices on a local segment using MAC addresses.
3. **Network:** Moves packets across multiple interconnected networks using logical IP addresses.
4. **Transport:** Provides end-to-end delivery between specific programs (such as via TCP and UDP).
5. **Session:** Manages and coordinates ongoing conversations between applications.
6. **Presentation:** Formats and encrypts data for presentation or transmission.
7. **Application:** Represents the software interface that users directly interact with, such as web browsers and email clients.

The OSI Seven-Layer Model

While the OSI model serves as the foundational conceptual framework for teaching, the Internet in practice uses the simpler **TCP/IP model**.

The TCP/IP model consolidates networking functions into four operational layers:
- **Link Layer:** Handles physical connections and direct data movement between adjacent devices.
- **Internet Layer:** Routes packets across multiple distinct networks using IP addressing.
- **Transport Layer:** Delivers data end-to-end between programs running on host machines.
- **Application Layer:** Encompasses the network applications and protocols that exchange user data directly.

The TCP/IP Model

Encapsulation is the process of packaging data as it moves through network layers. When a sending device prepares data to be transmitted across a network, the data moves downward through each layer of the model. At each stage, the current layer wraps the incoming data unit with its own specialized header containing protocol and addressing metadata.

Once the transmitted signal arrives at the destination device, the reverse process occurs. As the incoming data travels back upward through the layers, each layer reads the relevant control information and strips away its corresponding header before passing the underlying payload to the layer above.

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