Lesson 7.3: The Internet and How Data Travels

Introduction

In this lesson, we will explore the vast structure of the Internet, a global network that connects millions of devices and facilitates communication and information sharing. By the end of this lesson, students will understand how the Internet differs from a single network, the process that occurs when a browser requests a resource, and the principles of routing and cloud computing.

The Internet as Global Infrastructure

The Internet is an enormous network of networks which connects different devices and systems worldwide. Understanding the Internet's structure is crucial because it helps us comprehensively appreciate how data is transmitted across various nodes.

What Sets the Internet Apart?

The Internet differs from a single network in several ways:

• Infrastructure: The Internet consists of many interconnected but independent networks that operate on a decentralized model. In contrast, a single network typically refers to a local or proprietary system controlled by a single organization.
• Protocol: The Internet relies on common protocols to ensure data can travel across heterogeneous systems. The most notable of these is the Transmission Control Protocol/Internet Protocol (TCP/IP).
• Scalability: The Internet can grow without a centralized authority, allowing for the addition of networks and devices without major structural changes.

Example of Internet Structure

Consider how different networks, like a corporate network and a home WiFi network, connect to the Internet. Both can communicate and share data; however, they may use different internal protocols and security measures. When these networks connect with each other, they still adhere to the rules of TCP/IP, facilitating seamless communication.

The Process of Requesting a Resource

When a user enters a web address into a browser, a series of events occur to retrieve the desired resource. Understanding this process highlights the complex workings behind the seamless experience of browsing.

DNS Lookup

The first step in this process is the Domain Name System (DNS) lookup. The DNS translates human-readable domain names (like www.example.com) into IP addresses (like 192.0.2.1), which computers use to identify each other on the network.

1. User Action: students types a URL into the browser.
2. DNS Query: The browser checks its cache for the corresponding IP address. If it cannot find it, it sends a DNS query to a DNS resolver.
3. DNS Response: The resolver queries multiple servers until it finds the IP address, returning it to the browser.

Example of DNS Lookup

For example, if students wants to visit www.google.com:

• The browser first checks its local DNS cache.
• If the address is not cached, it queries its configured DNS server.
• The DNS server looks up and returns the IP address, which might be something like 172.217.14.206.

Request and Response Cycle

Once the IP address is obtained, the browser can then send an HTTP request to the server at that address.

1. Request: The browser sends an HTTP GET request to the server to retrieve the page.
2. Response: The server processes the request and sends back the requested resource, which is typically an HTML page.

Example of Request and Response

Continuing with the previous example of accessing www.google.com:

• The HTTP GET request is sent as follows:

$$

GET / HTTP/1.1

Host: www.google.com

$$

• The server then responds with a status code (like 200, indicating success) and the HTML data of the page.

Routing: How Data Finds Its Path

Routing refers to the process of determining the path that data packets take across multiple networks to reach their destination. This is crucial for effective Internet functionality.

How Routing Works

• Routers: Special devices that manage traffic on the Internet by directing data packets along the best paths.
• Routing Tables: Every router maintains a table that lists paths and the next hop for data packets. Routing protocols (like BGP, RIP) help maintain these tables and facilitate intercommunication among routers.

Real-World Routing Example

Consider sending a letter. The postal service uses a series of sorting facilities (routers) to determine the best route for the letter to reach its final destination. Similarly, data packets are routed through multiple routers, each deciding the best path based on current network conditions.

1. A data packet is created at students's device.
2. It travels to the nearest router.
3. The router uses its routing table to forward the packet to the next hop.
4. This process repeats until the packet reaches the destination server.

Cloud Computing: A Shift from Local to Hosted Services

Cloud computing represents a paradigm shift in how services are delivered over the Internet. Instead of relying solely on local servers or machines, users can access resources remotely from data centers.

Benefits of Cloud Computing

• Scalability: Users can access more resources as their needs grow without needing physical hardware.
• Cost Efficiency: Reduces the need for large upfront investments in hardware; users pay for what they use.

Example of Cloud Services

Consider a software application like Google Docs. Rather than installing the program on their local machines, users access it via the Internet, allowing for easy collaboration and access from any device with an Internet connection.

The Internet as a Network of Networks

Understanding that the Internet functions as a vast network of networks is crucial for appreciating its design and operation.

Addressing in the Internet

Each device on the Internet is assigned a unique address known as an IP (Internet Protocol) address, which allows it to be identified.

• IPv4 addresses are written as four decimal numbers separated by periods (e.g., 192.168.1.1).
• IPv6 addresses, which are longer and designed to accommodate the growing number of devices, are written as a series of hexadecimal numbers separated by colons.

Example of Addressing

When students sends a request to a website, the request includes the destination IP address, ensuring that the data packets reach the correct device. This is akin to including a return address on a letter to ensure it returns to the sender’s home.

Conclusion

In this lesson, we explored how the Internet, as a global infrastructure, functions and differs from a single network. We covered the process that occurs when a browser requests a resource, including DNS lookup, the request/response cycle, and routing mechanisms that allow data to travel efficiently across networks. We also discussed the transition to cloud computing and the significance of IP addressing as we consider the Internet's role as a network of networks.

Study Notes

• The Internet is a network of networks.
• Understanding DNS for domain resolution is crucial to web browsing.
• Routing is handled by routers using tables to determine optimal paths for data packets.
• Cloud computing offers scalable services remotely instead of relying on local infrastructure.
• IP addresses are essential for identifying devices on the Internet.