OSI Model

OSI Model

The OSI model explains how data travels through a network. It splits this process into **seven layers**. Each layer has its own job — from moving raw bits to showing a website.

Understanding the OSI model helps break down complex systems, and shows where tools, attacks, or failures happen.

OSI Layer Overview

The OSI model goes from Layer 1 (bottom) to Layer 7 (top). Each layer depends on the functionality of the ones above and below it. Knowing what happens at each layer helps in analyzing protocols, spotting weaknesses, and understanding where specific tools operate.

Layer	Number	What it does	Common Examples
Application	7	Interfaces with user applications and defines network services	HTTP, HTTPS, DNS, FTP, SMTP, IMAP, SSH, Telnet
Presentation	6	Translates, encrypts, and compresses data for the application layer	TLS/SSL, ASCII, JPEG, MPEG, GZIP, Base64
Session	5	Manages and maintains sessions between systems	NetBIOS, RPC, SMB, PPTP, SOCKS
Transport	4	Provides reliable or fast delivery of data streams	TCP, UDP, SCTP, QUIC
Network	3	Handles routing and logical addressing across networks	IP, ICMP, IGMP, IPsec, BGP, OSPF
Data Link	2	Transfers frames between directly connected devices using MAC addressing	Ethernet, ARP, PPP, VLAN (802.1Q), LLDP
Physical	1	Moves electrical or optical signals across physical media	Ethernet (cabling), Wi-Fi (802.11), fiber optics, hubs, radio signals

Easy Breakdown by Layer

Each layer in the OSI model has a specific role. Together, they describe how data moves across a network — from your device to another system, step by step.

Layer 1 – Physical

Moves raw bits (0s and 1s) over cables or air.

Involves physical elements like Ethernet cables, fiber optics, and Wi-Fi signals.

Example: When you plug in a network cable or connect to Wi-Fi, Layer 1 is active.

Layer 2 – Data Link

Connects devices on the same local network.

Uses MAC addresses to make sure data reaches the right device.

Example: Your router sends a frame to your laptop using its MAC address.

Layer 3 – Network

Routes data between networks.

Uses IP addresses to find the best path for packets.

Example: Sending a ping to a website shows Layer 3 in action.

Layer 4 – Transport

Ensures complete delivery of data.

Splits information into segments and reassembles them correctly.

Example: TCP guarantees a webpage loads fully, UDP is used for faster but less reliable connections like video calls.

Layer 5 – Session

Manages the start, duration, and end of communication sessions.

Keeps track of active connections between systems.

Example: Maintaining a stable connection while using SSH or video conferencing.

Layer 6 – Presentation

Makes data readable and secure.

Handles file formats, encryption, and compression.

Example: TLS secures your connection when using HTTPS, and JPEG ensures images display correctly.

Layer 7 – Application

Connects software to the network.

Defines how user-facing programs communicate over the network. Often includes a graphical user interface (GUI) for interacting with network services.

Example: Opening a website in a browser (HTTP), sending an email from an app (SMTP), or resolving a domain name using DNS.

What You Can Do with It

Understanding the OSI layers helps you:

• Know where a tool or attack operates
• Analyze traffic at the right level
• Identify weak points or misconfigurations
• Work smarter with packet captures, scans, or payloads

Protocols by Layer

Layer	Number	Function Summary	Common Protocols
Application	7	Interfaces with apps like browsers and mail	HTTP, FTP, DNS, SMTP
Presentation	6	Formats, encrypts, and compresses data	TLS/SSL, Base64, JPEG
Session	5	Manages connections and sessions	NetBIOS, PPTP, SMB
Transport	4	Ensures reliable or fast delivery	TCP, UDP
Network	3	Routes packets between networks	IP, ICMP, IPsec
Data Link	2	Sends frames on the local network	Ethernet, ARP
Physical	1	Transfers raw bits via cables or air	Wi-Fi (802.11), Ethernet (cabling)

See Also

• TCP/IP Stack
• IP Addressing
• MAC Addressing
• ARP Protocol
• Routing Basics