Topics Covered in This Blog

• What is the Internet?

• How Does the Internet Work?

• Network Types – LAN, MAN, WAN

• OSI Model (Open Systems Interconnection)

• TCP/IP Protocols

• Common Protocols in the Application Layer

• Recap

What is the Internet?

The Internet is a massive global network that connects millions of computers and devices worldwide.In simple terms, it’s a network of networks, working together using standardized communication protocols.

How Does the Internet Work?

• Optical fiber cables form the backbone of the internet — they carry huge amounts of data across countries and continents.

• These cables are laid under oceans and managed by Tier 1 companies (e.g., Tata Communications).

• Data flows like this:

  • Tier 1 companies → connect with Tier 2 providers

  • Tier 2 connects with ISPs (Internet Service Providers) like Jio, Airtel, Vodafone

  • Your home broadband connects to these ISPs — and that’s how the internet reaches you.

Question: Why Optical Fiber over Satellite?

• Satellite communication is slower due to higher distance.

• Optical communication is faster because the distance is shorter and data moves at the speed of light in fiber.

Network Types – LAN, MAN & WAN

LAN (Local Area Network)

• Scope: Inside a small area like your home, office, or school.

• Example: Wi-Fi network in your house.

MAN (Metropolitan Area Network)

• Scope: Covers a larger area than LAN, like a city or large campus.

• Example: Broadband network provided by Airtel in a city.

• Role: Acts as a bridge between LAN and WAN

WAN (Wide Area Network)

• Scope: Covers large geographical regions (multiple cities or countries).

• Example: Airtel connects different ISPs across the country — that's a WAN.

LAN ⟶ connects to MAN ⟶ connects to WAN

OSI Model – The 7 Layers of Networking

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to understand and standardize how different systems communicate over a network. It divides the process of data transmission into seven distinct layers, each with a specific role — from user-facing applications all the way down to physical hardware.

The 7 Layers (Top to Bottom)

1. Application Layer

2. Presentation Layer

3. Session Layer

4. Transport Layer

5. Network Layer

6. Data Link Layer

7. Physical Layer

Layer-7 Application Layer

• Purpose: Closest to the end-user; provides services like email, file transfer, and browsing.

• Functions: Interfaces directly with user applications.

• Example: When you visit https://github.com, your browser uses the Application layer to send and receive web data via HTTP/HTTPS.

Layer-6 Presentation Layer

• Purpose: Formats and translates data between the application and network.

• Functions: Handles encryption, decryption, compression, and data conversion.

• Example: When you open an encrypted PDF, this layer decrypts the data so the app can display it correctly.

Layer-5 Session Layer

• Purpose: Manages communication sessions between devices.

• Functions: Establishes, maintains, and terminates connections.

• Example: During a Zoom meeting, the session layer keeps the call active and stable until you disconnect.

Layer-4 Transport Layer

• Purpose: Ensures reliable and error-free delivery of data.

• Functions: Splits data into segments, handles re-transmissions and acknowledgments.

• Example: When downloading a file, the TCP protocol at this layer ensures you receive all chunks in order and without error.

Layer-3 Network Layer

• Purpose: Determines the best path for data to travel across networks.

• Functions: Handles IP addressing and routing.

• Example: Running ping 8.8.8.8 checks if your system can reach Google's DNS — the routing is handled at this layer.

Layer-2 Data Link Layer

• Purpose: Transfers data between devices on the same network.

• Functions: Uses MAC addresses, handles frame synchronization and error detection.

• Example: When your laptop connects to Wi-Fi, the Data Link layer ensures it talks to the correct router using MAC addresses.

Layer-1 Physical Layer

• Purpose: Deals with the actual physical connection between devices.

• Functions: Manages hardware like cables, signals, voltages, and radio frequencies.

• Example: Ethernet cables, Wi-Fi signals, and fiber optics all operate at this layer. If a cable is unplugged, the problem is here.

TCP/IP Model – The Core Protocol of Internet

While the OSI model is a theoretical framework, the TCP/IP model is what the internet and real-world networks actually use. It simplifies communication into just four layers, making it more practical and implementation-friendly

The 4 Layers (Top to Bottom)

1. Application Layer

2. Transport Layer

3. Internet Layer

4. Network Access Layer

Application Layer

• Purpose: Closest to the user; supports software applications and end-user services.

• Functions: Provides protocols for data exchange like HTTP, FTP, SMTP, DNS, etc.

• Example:

  • When you send an email, SMTP (Simple Mail Transfer Protocol) is used.

  • When you browse the web, your browser uses HTTP/HTTPS to communicate with websites.

  • DNS resolves www.google.com to its IP address at this layer.

Transport Layer

• Purpose: Responsible for reliable data delivery between devices.

• Functions: Segments data, handles flow control, error checking, and retransmission.

• Protocols Used:

  • TCP (Transmission Control Protocol) – Reliable, connection-oriented (e.g., file downloads).

  • UDP (User Datagram Protocol) – Faster but unreliable, connectionless (e.g., video streaming or online gaming).

• Example:

  • A large file download via HTTP uses TCP to ensure every byte reaches correctly.

  • A live cricket match stream on a sports app uses UDP to minimize delay even if a few packets are lost.

Internet Layer

• Purpose: Handles logical addressing and routing of data across networks.

• Functions: Assigns IP addresses, selects best routes, and packages data into IP packets.

• Key Protocols:

  • IP (Internet Protocol) – Defines addressing and routing.

  • ICMP (Internet Control Message Protocol) – Used for diagnostics like ping.

• Example:

  • When you use ping 8.8.8.8, ICMP sends a test packet and checks if Google’s DNS server is reachable.

  • IP determines the path your data takes from your computer to a server across the globe.

Network Access Layer (Link Layer)

• Purpose: Responsible for the physical transmission of data.

• Functions: Defines how data is sent over the hardware (like Ethernet or Wi-Fi).

• Protocols Used: Ethernet, Wi-Fi (IEEE 802.11), ARP (Address Resolution Protocol), etc.

• Example:

  • When you connect your laptop to a router over Wi-Fi, this layer governs the actual radio signal transmission.

  • ARP resolves an IP address to a MAC address so devices can find each other on a local network.

Common Protocols in the Application Layer

These protocols live in the Application Layer of both the OSI and TCP/IP models and help users interact with the internet and other services.

• HTTP

• FTP

• SMTP

• SMTP

1. HTTP – HyperText Transfer Protocol

• Purpose: Transfers web pages and data between your browser and a web server.

• Port Used: 80 (insecure), 443 for HTTPS (secure).

• Working:

  • A client (browser) sends a request to a server.

  • The server responds with the requested content (like HTML, images, or scripts).

2. FTP – File Transfer Protocol

• • Purpose: Transfers files between a client and server.

    • Port Used: 21 (control), 20 (data).

    • Working:

      • Connect to a server using an FTP client (e.g., FileZilla).

      • Upload/download files to/from the remote server.

    • Example:

      • Web developers use FTP to upload their website files to a hosting server.

      • DevOps engineers might use FTP to transfer logs or backups between systems.

    • Note: FTP is unencrypted by default. For secure transfer, SFTP (Secure FTP) or FTPS is used.

3. SMTP – Simple Mail Transfer Protocol

   • Purpose: Sends and relays email messages between mail servers.

   • Port Used: 25, 587 (with TLS).

   • Working:

     • Your email client (like Outlook or Gmail) sends an email to your SMTP server.

     • The SMTP server then relays that message to the recipient’s mail server.

   • Example:

     • When you hit "send" on an email, SMTP handles the delivery to the recipient’s server.

     • DevOps may configure SMTP settings in apps to enable email notifications or alerts.

   • Limitation: SMTP is only for sending emails. Receiving is handled by IMAP or POP3.

4. DNS – Domain Name System

   • Purpose: Resolves domain names into IP addresses.

   • Port Used: 53

   • Working:

     • You type www.google.com in your browser.

     • DNS converts it to 142.250.190.4 (an IP address your system can connect to).

   • Example:

     • Just like a phonebook maps names to numbers, DNS maps website names to IPs.

     • Without DNS, you'd have to remember the IP address of every website you visit!

Recap

Internet Fundamentals

• The Internet is a network of networks connecting devices globally.

• It works using optical fiber cables, managed by Tier 1, Tier 2, and local ISPs.

• Optical fibers are preferred over satellites for speed and reliability.

Network Types

• LAN: Local network (e.g., home or office Wi-Fi).

• MAN: City-wide network (e.g., Airtel broadband in a city).

• WAN: Country or global network (e.g., Airtel connecting ISPs nationwide).

OSI Model – 7 Layers

1. Application – User apps (e.g., browser).

2. Presentation – Encryption, formatting (e.g., SSL).

3. Session – Manages connections (e.g., Zoom call).

4. Transport – Reliable delivery (e.g., TCP).

5. Network – Routing & IP (e.g., ping).

6. Data Link – MAC-based delivery on local network.

7. Physical – Hardware & signals (e.g., cables, Wi-Fi).

TCP/IP Model – 4 Layers

1. Application – Protocols like HTTP, FTP, DNS, SMTP.

2. Transport – TCP/UDP for data delivery.

3. Internet – IP addressing and routing.

4. Network Access – Actual transmission over physical media.