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OOTW / Chapter I - Foundation / 03. Networking / 01. OSI Model

OSI Model

Before we start discussing protocols, routing, switching, and network attacks, we must first understand the conceptual framework used to describe how network communication works.

The OSI Model is one of the most important concepts in networking.

It provides a structured way to think about communication between systems by dividing networking responsibilities into separate layers.

OSIModel

Why The OSI Model Exists

Imagine trying to troubleshoot a network problem without any structure.

A user says:

"The website doesn't load."

What does that mean?

  • Cable unplugged?
  • Switch failure?
  • Wrong IP?
  • DNS failure?
  • Firewall blocking traffic?
  • Web server offline?

Without a framework, troubleshooting becomes chaos.

The OSI Model solves this by dividing networking into layers.

Each layer is responsible for a specific part of communication.

Core Principle

Each layer provides services to the layer above it.

Layer 7
    ↓
Uses Layer 6

Layer 6
    ↓
Uses Layer 5

Layer 5
    ↓
Uses Layer 4

...

No layer needs to know how the lower layers actually work.

It only uses the services they provide.

Layer 1 — Physical

The Physical Layer is responsible for transmitting raw bits.

It deals with:

  • Electrical signals
  • Light signals
  • Radio waves
  • Cables
  • Connectors

Examples:

Ethernet Cable
Fiber Optic Cable
Wi-Fi Radio Signal

The Physical Layer does not understand:

IP Addresses
Ports
Protocols
Packets

It only moves bits.

Unit of data:

01100111 01101111 00100000 01100010 01100001 01100011 01101011 00100000 01110100 01101111 00100000 01100011 01101100 01100001 01110011 01110011 00100000 01101100 01101111 01101100

Layer 2 — Data Link

The Data Link Layer is responsible for communication between devices on the same local network.

Its primary job is framing and MAC addressing.

Example:

PC → Switch

Layer 2 identifies devices using:

MAC Addresses

Example:

00:11:22:33:44:55

Common technologies:

Ethernet
Wi-Fi

Devices operating primarily at Layer 2:

Switches
Bridges

Unit of data:

Frame

Layer 3 — Network

The Network Layer is responsible for routing traffic between networks.

This is where IP addresses live.

Example IPv4:

192.168.1.10

or IPv6:

2001:db8::1

Layer 3 determines:

How do I reach another network?

Devices:

Routers
Layer 3 Switches

Protocols:

IPv4
IPv6
ICMP

Unit of data:

Packet

Layer 4 — Transport

The Transport Layer is responsible for end-to-end communication between applications.

Its responsibilities include:

  • Reliability
  • Flow Control
  • Error Recovery
  • Segmentation

The two most important protocols are:

TCP
UDP

TCP:

Reliable
Connection-oriented
Acknowledgements
Retransmissions
Ensures the traffic is received

Example:

Web Browsing → TCP

UDP:

Fast
Connectionless
No reliability guarantees
Sends traffic without ensuring it is received

Examples:

Video Streaming → UDP
VoIP → UDP

Unit of data:

Segment

Layer 5 — Session

The Session Layer manages communication sessions between systems.

Responsibilities:

Session Establishment
Session Maintenance
Session Termination

Example:

User logs into a remote application

A session is created and maintained.

Modern networking often merges Session functionality into higher layers.

Because of this, Layer 5 is rarely discussed independently in real-world environments.

Layer 6 — Presentation

The Presentation Layer is responsible for data representation.

It answers:

How should data be formatted?

Responsibilities:

Encryption
Compression
Encoding
Translation

Examples:

TLS/SSL
ASCII
UTF-8
JPEG
PNG

Example:

Plain Text
      ↓
TLS Encryption
      ↓
Encrypted Data

Layer 7 — Application

The Application Layer is the closest layer to the user.

This is where applications interact with network services.

Examples:

HTTP
HTTPS
FTP
DNS
SMTP
SSH

Applications operating here:

Chrome
Firefox
Outlook
curl
wget

Important:

The Application Layer is not the application itself.

It is the networking functionality used by applications.

Encapsulation

One of the most important networking concepts is encapsulation.

As data moves down the OSI stack, each layer adds information.

Example:

Application Data

Layer 4 adds:

TCP Header

Layer 3 adds:

IP Header

Layer 2 adds:

Ethernet Header

Result:

[Ethernet]
    [IP]
        [TCP]
            [Data]

This process is called:

Encapsulation

The receiving host performs:

De-encapsulation

removing headers layer by layer.

Real Example — Loading A Website

You open:

https://example.com

Application Layer:

HTTP Request

Presentation Layer:

TLS Encryption

Transport Layer:

TCP Segment

Network Layer:

IP Packet

Data Link Layer:

Ethernet Frame

Physical Layer:

Electrical Signals

The server receives the traffic and reverses the process.

What Security Professionals Care About

Different attacks occur at different layers.

Layer 2:

ARP Spoofing
MAC Flooding

Layer 3:

IP Spoofing
Routing Attacks
ICMP Abuse

Layer 4:

Port Scanning
TCP Hijacking
SYN Flooding

Layer 7:

SQL Injection
XSS
Authentication Bypass
API Abuse

Understanding the OSI Model helps us quickly determine:

Where a problem exists.
Where an attack occurs.
Where a defense should be implemented.

Operator Notes

The OSI Model is not a perfect representation of modern networking.

Many modern protocols blur the boundaries between layers.

However, it remains one of the most useful conceptual frameworks for:

  • Understanding network communication
  • Troubleshooting connectivity problems
  • Learning protocols
  • Understanding attacks and defenses

For offensive operators and defenders alike, the OSI Model provides a mental map that explains where data exists, how it moves, and where it can be observed, manipulated, or protected.