1 OSI Model

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💡 Main idea: OSI model is used to understand how data is Transfer from one computer to another.

OSI is theoretical (used for understanding),
TCP/IP is practical (used in real-world networking).

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⚙️ 2️⃣ Layer-wise Explanation

🧩 OSI Model (7 Layers)

🧱 Layer	💡 Purpose in Real World	⚙️ Examples / Where You See It	📦 Packet Name (PDU)
7️⃣ Application	Interface for end-users & apps to use the network	Web browsers (Chrome), APIs, Mail Clients	Data
6️⃣ Presentation	Converts data format, handles encryption & compression	SSL/TLS during HTTPS, JSON ↔ Binary conversions	Data
5️⃣ Session	Maintains communication sessions (login/auth, tokens)	WebSocket’s, Remote logins, Authentication sessions	Data
4️⃣ Transport	Reliable delivery, segmentation, flow & error control	TCP for web apps, UDP for streaming	Segment (TCP) / Datagram (UDP
3️⃣ Network	Handles addressing & routing between networks	IP, Routers, Firewalls	Packet
2️⃣ Data Link	Moves frames within local network, MAC addressing	Switches, Ethernet, Wi-Fi	Frame
1️⃣ Physical	Actual transmission of bits over cable or air	Hubs, Fiber, Ethernet cable, Wi-Fi signals	Bits

🌐 TCP/IP Model (4 Layers)

🌐 TCP/IP Layer	🧱 Mapped OSI Layers	💡 Purpose in Real World	⚙️ Common Protocols / Examples
Application	OSI (7, 6, 5)	Interface for user apps, data formatting, sessions	HTTP, HTTPS, DNS, SSH, FTP, SMTP
Transport (TCP)	OSI (4)	Reliable or fast delivery between systems	TCP, UDP
Internet (IP)	OSI (3)	Logical addressing & routing between networks	IP
Network Access (Physical)	OSI (2, 1)	Physical transmission & local network communication	Ethernet, Wi-Fi

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“In practice, we troubleshoot based on TCP/IP layers”

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🚀 server.js - Express.js Server (Real-world TCP/IP Example)

// ==============================================
// server.js — Simple Express.js Web Server
// ==============================================
 
// 🧠 Application Layer — Our code lives here
const express = require("express");
const app = express();
 
// Middleware to parse JSON requests (optional)
app.use(express.json());
 
 
// ==============================================
// 1️⃣ Application Layer
// ----------------------------------------------
// This is where the actual web application logic lives.
// Express handles HTTP requests and responses.
// HTTP itself is an Application Layer protocol built on TCP.
// ==============================================
app.get("/", (req, res) => {
  res.send("Hello from the Express.js Server 🌐");
});
 
 
// ==============================================
// 2️⃣ Transport Layer (TCP)
// ----------------------------------------------
// When Express starts listening, Node.js creates a TCP socket
// (via the OS) to handle reliable communication between
// client and server.
// - TCP ensures reliability, retransmission, and flow control.
// ==============================================
const PORT = 3000;
app.listen(PORT, () => {
  console.log(`🚀 Server is running at http://localhost:${PORT}`);
});
 
 
// ==============================================
// 3️⃣ Internet Layer (IP)
// ----------------------------------------------
// Under the hood, your OS binds the app to an IP address
// like 127.0.0.1 (localhost) or your LAN IP (192.168.x.x).
// IP handles routing packets between devices on different networks.
// ==============================================
 
 
// ==============================================
// 4️⃣ Network Access Layer
// ----------------------------------------------
// This is where the data is actually transmitted through
// physical hardware (Ethernet/Wi-Fi).
// It includes MAC addressing and low-level frame transmission.
// ==============================================

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💡 How It Fits into the TCP/IP Model

TCP/IP Layer	Real Example in Express
Application Layer	Express.js app (app.get(...), res.send(...))
Transport Layer	TCP connection created by Node’s HTTP module
Internet Layer	IP routing (e.g., 127.0.0.1 or 192.168.1.x)
Network Access Layer	Ethernet or Wi-Fi sending physical frames

🔄TCP ⚡UDP