🧠 The OSI Model

OSI (Open Systems Interconnection) Model is a framework that standardizes how data is transferred between two systems over a network.

It has 7 layers, each with a specific role in ensuring smooth, secure, and reliable communication.

🎮 Imagine This:

Two friends, A and B, are playing a multiplayer online game from different countries.

Let’s use this scenario to understand how the OSI Model works.

📚 OSI Layers Mnemonic:

All People Seem To Need Data Processing

(Application → Physical)

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🔼 Layer 7: Application Layer – “Let’s Play!”

This is where A and B run the actual game app (e.g., PUBG, Fortnite).

• This is the interface between the user and the network.

• They use a browser or mobile app.

• Protocols: HTTP, HTTPS, FTP, SMTP

💡 In DevOps, you work with these protocols while managing servers and web apps.

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🎨 Layer 6: Presentation Layer – “Graphics, Sounds, and Security”

A and B are seeing graphics, using keyboard, listening to game sounds, and the data is encrypted.

• It manages how the data is presented: format, encoding, encryption.

• It converts data to be readable by the receiver.

• Also responsible for SSL/TLS encryption (important in HTTPS).

🛡️ Think of this as the syntax and security formatting of the data.

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🔌 Layer 5: Session Layer – “Let’s Connect”

Before playing, a connection is established between A and B (like a game lobby).

• This layer establishes, maintains, and ends communication sessions.

• Responsible for authentication and session management.

🔧 Examples:

API sessions: when apps talk to each other (e.g., payment gateways).

Sockets: used for real-time communication like chat, games, etc.

👉 API: Bridge between two apps (e.g., Google Maps inside Zomato app)

👉 Socket: Real-time data channel (e.g., online multiplayer game, live chat)

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📦 Layer 4: Transport Layer – “Send It Reliably”

The game data like movements, actions, chat is now broken into packets and sent between A and B.

• Responsible for reliable delivery, error checking, and flow control.

Protocols:

TCP (Transmission Control Protocol) – Reliable, connection-based

UDP (User Datagram Protocol) – Faster but no guarantee (used in live games, video calls)

🧪 You’ll hear this in DevOps when setting up ports, firewalls, or troubleshooting connection issues.

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🛰️ Layer 3: Network Layer – “Find the Way”

A and B’s data has to travel across the globe. So it needs routing.

• Responsible for IP addressing and routing.

• It finds the best path for data.

Protocol: IP (Internet Protocol)

📍 Example: Your request goes from India to the US via routers using IPs.

You can trace this using:

traceroute google.com    # Linux/macOS
tracert google.com      # Windows

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🔗 Layer 2: Data Link Layer – “Local Delivery”

Now the data is being transferred within the same local network (e.g., from your laptop to your WiFi router).

• Deals with MAC addresses, frames, and error detection.

• Ensures data is sent to the right device on a local network.

👀 You can check your MAC address using:

ip link show

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🧱 Layer 1: Physical Layer – “The Real Wires”

Finally, all of this data flows through real hardware – like fiber optic cables, network cards, or WiFi signals.

• Transmits 0s and 1s (binary data) over physical medium (cables, radio waves).

Includes: Cables, Switches, Hubs, Connectors, NICs.

💡 Without this, the internet wouldn’t exist physically.

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🔄 Software vs Hardware in OSI Model

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🧵 Final Summary: Gaming the OSI Model

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✅ Ready for DevOps?

OSI is more than theory. You’ll use it when:

• Configuring ports (Layer 4)

• Understanding IP routing and DNS (Layer 3)

• Managing VPNs and proxies

• Debugging with ping, traceroute, netstat, tcpdump, etc.