WORKING OF INTERNET

HOW Does Internet Work

1. The Need to Share Information
   • Core Idea: Transfer data from one computer (A) to another (B).
   • Primitive Method: Physical transfer (USB) — slow and not scalable.
   • Solution: Direct electronic connection via cables (Ethernet/Fiber).
   • Concept Introduced: Direct Link — allows transmission of bits (1s and 0s).


2. The Problem of scale
   • Issue: Direct links between every pair of computers become impractical as numbers grow.
   • Example: 100 computers need 4,950 cables!
   • Solution: Introduce a Switch — central device that connects all computers.
   • Concept Introduced: Local Network (LAN) — efficient communication within a group.


3. Connecting Multiple Networks
   • Scenario: Two offices with separate LANs need to communicate.
   • Problem: Can't connect every computer across networks.
   • Solution: Use a Router — connects different networks and routes traffic.
   • Concept Introduced: Internet = Network of Networks — routers interconnect LANs globally.


4. Addressing: Finding the Right device
   • Challenge: Identifying the destination among millions of devices.
   • Solution: Assign a unique IP Address to every device (e.g., 142.250.184.142).
   • Concept Introduced: IP Addressing — essential for routing messages correctly.


5. Reliability and fairness
   • Problem: Sending large files monopolizes bandwidth and risks corruption.
   • Solution: Break data into small units called Packets.
   • Each packet contains:
     1. Payload (data)
     2. Destination IP
     3. Sender IP
     4. Sequence Number
   
   
   • Packets travel independently and are reassembled at the destination.
   • Concept Introduced: Protocol — rules for data transfer. Most common: TCP/IP (Transmission Control Protocol / Internet Protocol)

NETWORKING ESSENTIALS

1. IP Address(Internet Protool)
   • IPv4: 32-bit address, written as four decimal numbers (0–255). Example: 172.217.16.142 Limit: ~4.3 billion addresses → led to scarcity.
   • Public IP: Globally unique. Assigned by ISP. Used for internet-facing devices (e.g., servers, routers).
   • Private IP: Used within local networks. Assigned by router. Can be reused across different networks. Reserved Ranges: 10.0.0.0 – 10.255.255.255 172.16.0.0 – 172.31.255.255 192.168.0.0 – 192.168.255.255
2. IPv6 (Internet Protocol version 6)
   • 128-bit address → 2¹²⁸ ≈ 340 undecillion addresses.
   • Format: Hexadecimal, colon-separated. Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
   • Purpose: Solves IPv4 exhaustion.
3. MAC Address (Media Access Control)
   • 48-bit hardware address burned into network devices.
   • Formats: Colon-separated: 3C:22:FB:A3:B4:C5 Hyphen-separated: 3C-22-FB-A3-B4-C5 Period-separated: 3c22.fba3.b4c5 No separators: 3C22FBA3B4C5
   • Use: Device-level identification on local networks.
4. Port Numbers
   • 16-bit unsigned integer → Range: 0–65535
   • Purpose: Identify specific services/applications on a device.

   • Port Number Ranges

     Category	Range	Description
     Well-Known Ports	0–1023	Reserved for standard services (e.g., HTTP, FTP, SSH)
     Dynamic/Ephemeral Ports	49152–65535	Temporary ports used by clients for outgoing connections

Key Takeaways for Interview

• IPv4 vs IPv6: Know the difference in size and format.
• Public vs Private IP: Understand scope and assignment.
• MAC Address: Hardware-level identity.
• Port Numbers: Know categories and key examples.
• Reserved IP ranges and port privileges (e.g., root access for ports less than 1024).

DNS(DOMAIN NAME SYSTEM)

What is DNS?

1. Local Cache Lookup
   Before querying the internet,your system checks:
   1. Browser Cache – Recently visited sites.
   2. OS Cache – Stored by your operating system.
   3. Router Cache – Some routers store DNS responses.
2. Recursive DNS Reslover
   • If not cached, the system queries a Recursive Resolver (usually from your ISP or public DNS like 8.8.8.8 or 1.1.1.1).
   • Recursive means it will do all the work to find the final IP by navigating the DNS hierarchy.
3. Hierarchical DNS Lookup

   • Level	Server Type	Role
     1	Root Server	Directs to TLD servers (e.g., .com)
     2	TLD Server	Directs to authoritative servers for domain (e.g., google.com)
     3	Authoritative Name Server	Provides the actual IP address for the domain

4. Returning the Answer
   1. Authoritative Server → Recursive Resolver
   2. Resolver caches the result (based on TTL)
   3. Resolver → Operating System
   4. OS → Browser → Website loads

Flow Diagram:-