[Notes] Computer Network

What is the Internet?

The Internet is a global network of interconnected computers that communicate using standard protocols (rules). It’s not a single entity but a network of networks that allows millions of devices worldwide to exchange data.

In simpler terms, the Internet is like a huge web connecting different smaller networks, allowing information to flow from one part of the world to another.

Layers of the Internet Structure

The Internet is organized into several layers, each with specific responsibilities. These layers form the Internet protocol stack (also called the TCP/IP model), which enables data transmission.

Here’s a simplified view of these layers:

Physical Layer: The actual hardware that carries data (e.g., cables, fiber optics, Wi-Fi).
Link Layer: Manages communication between directly connected devices. Examples: Ethernet, Wi-Fi.
Network Layer: Routes data from the source to the destination across multiple networks. The most common protocol used here is the Internet Protocol (IP).
Transport Layer: Ensures reliable data transfer between two systems. Common protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Application Layer: Consists of the applications and protocols that users interact with, like HTTP (for web browsing), FTP (for file transfer), and SMTP (for email).

Key Components of the Internet

The Internet is made up of several key components that allow it to function efficiently:

1. End Systems (Hosts)

These are the devices at the “edges” of the network, like your computer, smartphone, or tablet. They can be classified into:

Clients: Devices that request services (e.g., web browsers).
Servers: Devices that provide services (e.g., web servers, file servers).

2. Routers

Routers are devices that forward data between networks. They make sure data gets from its source to its destination by choosing the best path across the Internet, even if multiple networks are involved.

3. Communication Links

These are the physical and wireless connections that carry data between devices. Links can be:

Wired Links: Ethernet cables, fiber optics.
Wireless Links: Wi-Fi, LTE, 5G.

4. Internet Service Providers (ISPs)

ISPs provide access to the Internet for end systems. They can be categorized into three tiers:

Tier 1 ISPs: Large global networks that connect to each other and provide access to the whole Internet without paying anyone else. They form the backbone of the Internet.
Tier 2 ISPs: Regional networks that connect to Tier 1 ISPs.
Tier 3 ISPs: Local ISPs that provide Internet access to homes, businesses, and smaller regions.

5. Protocols

Protocols are sets of rules that define how data is transferred over the Internet. Some of the most important protocols are:

IP (Internet Protocol): Defines how data is broken into packets and routed across networks.
TCP (Transmission Control Protocol): Ensures reliable transmission of data.
UDP (User Datagram Protocol): Provides faster, but less reliable, data transmission.
DNS (Domain Name System): Translates human-readable domain names (like google.com) into IP addresses.

How Data Travels on the Internet

When you send data (like loading a webpage), this is how it travels through the Internet:

Your Device (End System): You enter a URL in your browser. Your computer sends a request for the webpage to the web server.
DNS Lookup: Your computer doesn’t know where google.com is, so it uses the Domain Name System (DNS) to find the IP address of the server hosting the webpage.
Transport Layer: The data is broken into smaller pieces called packets. The TCP protocol ensures all packets are sent in the right order and none are lost.
Network Layer: Each packet is given an IP address to specify its source and destination. Routers forward these packets across different networks.
Link Layer: The physical communication links (wired or wireless) carry the packets between routers and devices.
Web Server: The packets reach the web server, and the server sends the requested webpage back to your computer in the same way.

A Real-World Example

Let’s consider what happens when you access www.google.com:

Request: Your browser sends a request to the DNS server to get the IP address of www.google.com.
Routing: Once the IP address is found, your request is broken into packets and sent via routers through various networks until it reaches Google’s web server.
Response: Google’s server processes the request and sends the webpage back in the form of packets.
Reassembly: Your browser reassembles the packets to display the webpage you requested.

The Role of IP Addresses and Domain Names

IP Address: Every device on the Internet has a unique IP address (e.g., 192.168.0.1), much like a postal address for sending data.
Domain Name: Since IP addresses are hard to memorize, human-readable domain names (e.g., www.google.com) are used. The DNS translates these domain names into IP addresses.

Internet Architecture Summary

End Systems (clients and servers) communicate with each other using routers to forward data.
ISPs connect different networks together, forming the backbone of the Internet.
Protocols like IP, TCP, UDP, and DNS ensure data is transmitted correctly and efficiently.
Data is broken into packets, routed through multiple networks, and reassembled at the destination.