How The Internet Works — Beginner Friendly System Design Guide

You Opened Swiggy. What Actually Happened?

You tap the Swiggy app.

Within milliseconds:

• your phone talks to DNS servers
• connects to internet providers
• reaches Swiggy servers
• fetches restaurant data
• downloads images
• renders UI

All before you even blink.

That “instant loading” feeling is not magic.

It is:

• networking
• protocols
• servers
• caching
• distributed systems

working together.

---

Why This Topic Matters

Before learning:

• microservices
• databases
• load balancing
• distributed systems

you must first understand:

How computers communicate over the internet

Every system design concept depends on this foundation.

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The Internet Is Just Computers Talking

Very simple idea:

Internet = Millions of computers communicating

Just like humans need:

• address
• language
• delivery system

computers also need:

• IP addresses
• protocols
• network routing

---

Real Example — Opening Zomato

Suppose you open:

zomato.com

What happens behind the scenes?

---

High-Level Flow

User Opens Zomato
        │
        ▼
DNS Finds IP Address
        │
        ▼
Browser Connects To Server
        │
        ▼
HTTPS Secure Connection
        │
        ▼
HTTP Request Sent
        │
        ▼
Server Processes Request
        │
        ▼
Database + Cache Access
        │
        ▼
Response Returned
        │
        ▼
App/UI Rendered

---

Step 1 — DNS Resolves Domain Name

Humans remember:

zomato.com

Computers use:

142.xxx.xxx.xxx

called IP addresses.

---

What Is DNS?

DNS stands for:

Domain Name System

It works like internet's phonebook.

---

Real-Life Analogy

Instead of remembering:

• everyone's phone number

you save:

• names in contacts

DNS does same thing.

---

DNS Flow

User Types zomato.com
        │
        ▼
Browser Checks Cache
        │
        ▼
DNS Resolver Asked
        │
        ▼
DNS Returns IP Address
        │
        ▼
Browser Connects To Server

---

Step 2 — TCP Connection

Now browser knows:

• where server exists

Next:

• connection must be established

using:

TCP

---

What Is TCP?

TCP means:

Transmission Control Protocol

It ensures:

• packets arrive correctly
• data remains ordered
• lost packets are retried

---

Real-Life Analogy

Imagine sending pages of book through courier.

TCP ensures:

• no pages missing
• pages stay in correct order

---

TCP Three-Way Handshake

Before sending data:

client and server establish connection.

---

TCP Handshake Flow

Client                Server
   │                     │
   │ ---- SYN ---------> │
   │                     │
   │ <--- SYN-ACK ------ │
   │                     │
   │ ---- ACK ---------> │
   │                     │
Connection Established

---

Why Handshake Matters

Every connection adds latency.

Approximate latency:

Route	Latency
Delhi → Mumbai	20-40 ms
India → Singapore	40-60 ms
India → USA	180+ ms

Physics matters.

---

Step 3 — HTTPS Secure Connection

Modern websites use:

HTTPS

instead of HTTP.

---

Why HTTPS Exists

Without encryption:

• passwords visible
• payments exposed
• tokens stealable

especially on public WiFi.

---

HTTPS Uses TLS

TLS encrypts communication.

---

HTTPS Flow

Browser Connects
       │
       ▼
Server Sends Certificate
       │
       ▼
Browser Verifies Certificate
       │
       ▼
Encryption Keys Generated
       │
       ▼
Secure Communication Starts

---

Lock Icon Meaning

The browser lock icon means:

Connection is encrypted

NOT:

• website is trustworthy

---

Step 4 — HTTP Request

Now browser sends request.

Example:

GET /restaurants HTTP/1.1
Host: zomato.com

---

Server Responds

HTTP/1.1 200 OK
Content-Type: application/json

with restaurant data.

---

Important HTTP Status Codes

Code	Meaning
200	Success
301	Redirect
400	Bad request
401	Unauthorized
403	Forbidden
404	Not found
500	Server error
503	Service unavailable

---

Step 5 — Data Travels As Packets

Internet does NOT send one giant file.

Data is broken into:

Packets

Small chunks of data.

---

Example

A 5 MB image may become:

• thousands of packets

Packets may:

• take different routes
• arrive out of order
• get lost

TCP handles reconstruction.

---

Packet Flow

Large File
    │
    ▼
Split Into Packets
    │
 ┌──┼──┬──┐
 ▼  ▼  ▼  ▼
P1 P2 P3 P4
    │
    ▼
Internet Routing
    │
    ▼
Packets Reassembled

---

Step 6 — CDN Serves Static Content

Images/videos usually come from:

CDN

instead of main server.

---

What Is CDN?

CDN means:

Content Delivery Network

Servers distributed globally.

---

Example — Hotstar IPL Streaming

If Jaipur user watches IPL:

• content served from nearby Indian CDN server
• not USA server

Much faster.

---

CDN Flow

User
  │
  ▼
Nearest CDN Server
  │
  ├── Cached Content Exists
  │         │
  │         ▼
  │    Return Fast
  │
  └── Not Cached
            │
            ▼
       Origin Server

---

Step 7 — Backend Server Processing

Application server may:

• authenticate user
• fetch restaurants
• calculate offers
• check Redis cache
• query database

---

Backend Architecture Example

Mobile App
    │
    ▼
Load Balancer
    │
    ▼
Application Servers
    │
 ┌──┴─────────┐
 ▼            ▼
Redis       Database
Cache

---

What Is UDP?

Not all internet traffic uses TCP.

Some use:

UDP

---

TCP vs UDP

TCP	UDP
Reliable	Faster
Ordered delivery	No guarantee
Retries packets	No retries
Higher latency	Lower latency

---

Real Examples

Application	Protocol
Websites	TCP
Payments	TCP
WhatsApp Messages	TCP
Video Calls	UDP
Gaming	UDP
DNS	Usually UDP

---

Why Video Calls Prefer UDP

In video calls:

• speed matters more than perfection

Better:

• small glitch

than:

• delayed conversation

---

Real Internet Journey — Swiggy Example

User Opens Swiggy
        │
        ▼
DNS Resolves Domain
        │
        ▼
TCP Connection Created
        │
        ▼
TLS/HTTPS Handshake
        │
        ▼
HTTP Request Sent
        │
        ▼
Load Balancer
        │
        ▼
Application Server
        │
 ┌────┼─────┐
 ▼          ▼
Redis      Database
        │
        ▼
Response Returned
        │
        ▼
UI Rendered

---

Why Internet Speed Feels Slow Sometimes

Possible bottlenecks:

Problem	Example
Slow DNS	Domain lookup delayed
High latency	Server far away
Packet loss	Weak network
Slow backend	Database issue
CDN miss	Fetching from origin
TLS overhead	Secure connection setup

---

Common Beginner Misconceptions

---

"Internet Is Cloud Magic"

No.

It's:

• routers
• cables
• servers
• protocols
• data centers

working together.

---

"HTTPS Means Safe Website"

No.

HTTPS only means:

• encrypted connection

Website itself may still be malicious.

---

"Database Is Always Slow Part"

Not always.

Sometimes issue is:

• DNS
• network
• frontend
• TLS handshake
• CDN

Always profile full request path.

---

Key Takeaways

Concept	Purpose
DNS	Domain → IP
TCP	Reliable communication
HTTPS/TLS	Encryption
HTTP	Request-response protocol
CDN	Faster content delivery
Packets	Small transferable data chunks
Load Balancer	Traffic distribution

---

Final Mental Model

Internet
   │
   ▼
DNS
   │
   ▼
TCP Connection
   │
   ▼
HTTPS Encryption
   │
   ▼
HTTP Request
   │
   ▼
Load Balancer
   │
   ▼
Application Server
   │
 ┌─┴─────────┐
 ▼           ▼
Cache      Database
   │
   ▼
Response Returned

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One-Line Interview Definition

The internet works through interconnected networks where clients and servers communicate using protocols like DNS, TCP/IP, HTTPS, and HTTP to exchange data reliably across the globe.