The OSI Model and Encapsulation
The OSI model and encapsulation concepts
What you will learn
- Order the seven layers from Physical to Application, and state each layer's job in one sentence.
- Distinguish each layer's data unit: bits, frames, packets, segments.
- Explain encapsulation and de-encapsulation as a reverse process between sender and receiver.
- Identify which layer common devices operate at: switch, router, firewall.
The OSI model isn't literally how the internet runs — real networks use the simpler 4-layer TCP/IP model. But OSI remains the shared language for describing problems: "is this a layer 2 or layer 3 issue?" is a question any network engineer hears daily, which is why the exam demands knowing it precisely.
The seven layers, bottom to top
1. Physical
Cables and electrical/optical signals — raw bits only.
2. Data Link
MAC addresses, frames — where a switch operates.
3. Network
IP addresses, packets — where a router operates.
4. Transport
TCP and UDP, segments, port numbers.
5. Session
Opening, managing, and ending a connection session.
6. Presentation
Data formatting, encryption, compression (e.g., TLS).
7. Application
What the user actually interacts with: HTTP, DNS, email.
Encapsulation on send
Each layer wraps its own header around the layer above's data — the exact reverse happens on receipt.
Application data
+ Transport header → Segment
+ Network header → Packet
+ Data Link header → Frame
Raw bits on the wire
Protocol Data Unit per layer
| Bits | Physical layer — raw signals. |
|---|---|
| Frame | Data Link layer — carries MAC addressing. |
| Packet | Network layer — carries IP addressing. |
| Segment | Transport layer — carries the port number (TCP/UDP). |
The most common trap
A question describes a switch, then asks "which layer does it operate at?" — the answer is always layer 2, because a basic switch only reads MAC addresses. A Layer 3 switch is an exception the question states explicitly, never assumed by default.
What matters on the exam
- Order the layers fast: Physical, Data Link, Network, Transport, Session, Presentation, Application.
- Switch = layer 2, Router = layer 3, unless the question states otherwise explicitly.
- TCP and UDP are both layer 4 — they differ in delivery reliability, not in layer.
Quick check
Answer in your head first, then reveal.
Which layer does a basic switch operate at?Reveal the answer
Data Link layer (layer 2).
What is the data unit at the Network layer?Reveal the answer
Packet.
What is the reverse of encapsulation on receipt called?Reveal the answer
De-encapsulation.
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Your next step
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Sources
Used to verify the facts. The writing is original to Passuit.