Cambridge Syllabus Notes

The Internet and Its Uses

Objective

Describe how a web page is located, retrieved and displayed on a device when a user enters a URL.

Internet vs. World‑Wide Web

Internet: The global network of interconnected computers, routers, and other devices that carries many kinds of data.
World‑Wide Web (WWW): A service that runs on the Internet; it is the collection of hyper‑text documents (web pages) accessed using the HTTP/HTTPS protocols.

Overall Process

When a URL is entered in a browser and Enter is pressed, the following sequence occurs:

URL parsing
DNS lookup (domain → IP address)
Establish a reliable TCP (or TLS) connection
Send an HTTP/HTTPS request
Receive the server’s response
Render the page in the browser

Web‑Browser Functions

Interpret and render HTML, CSS and JavaScript.
Manage URLs entered in the address bar and generate HTTP/HTTPS requests.
Store bookmarks/favourites and maintain a browsing history.
Handle cookies (see the “Cookies” section).

Step‑by‑Step Details

1. URL Parsing

The browser splits the URL into four parts (as required by the syllabus):

Protocol – e.g. http or https (defines the communication scheme).
Domain name – the human‑readable host, e.g. www.example.com.
Path – the location of the resource on the server, e.g. /articles/tutorial.html.
Query string – optional parameters after “?” that can affect the returned content, e.g. ?id=25&lang=en.

2. DNS Lookup

To translate the domain name into an IP address the browser follows this hierarchy:

Browser cache
Operating‑system cache
Configured DNS resolver (usually supplied by the ISP)
Authoritative name servers – the resolver contacts them until the IPv4 or IPv6 address is obtained.

3. TCP / TLS Connection

A reliable TCP connection is required before any data can be transferred.

TCP three‑way handshake (SYN → SYN‑ACK → ACK) establishes the connection.
If the URL uses https, a TLS handshake follows immediately, providing:
- Encryption (confidentiality)
- Server authentication via a digital certificate
- Integrity checking of the transferred data

4. HTTP / HTTPS Request

The browser builds an HTTP request line and a set of headers. Example for https://www.example.com/articles/tutorial.html:

GET /articles/tutorial.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9

Purpose of the protocols: HTTP transfers data in clear text; HTTPS (HTTP over TLS) adds encryption, authentication and integrity.

5. Server Response

The server replies with a status line, response headers and the requested content (or an error code). Example of a successful response:

HTTP/1.1 200 OK
Date: Tue, 16 Nov 2025 12:34:56 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 4523

<!DOCTYPE html>
<html>
    … page markup …
</html>

Common status codes required for the syllabus:

200 OK – request succeeded.
301/302 Redirect – resource moved to a new URL.
404 Not Found – resource does not exist.
500 Internal Server Error – problem on the server side.

6. Rendering the Page

After the response is received, the browser’s rendering engine processes the data in four stages:

Parse the HTML to build the Document Object Model (DOM) tree.
Parse CSS and apply styles to the DOM elements.
Execute JavaScript, which may modify the DOM or request further resources.
Paint the final layout onto the screen.

Cookies

A cookie is a small piece of data that a web server sends to a browser and that the browser stores for later use.

Session cookie – stored only in memory and deleted when the browser is closed. Example: a temporary login token.
Persistent cookie – written to the device’s storage with an expiry date and retained across browser sessions. Example: a “remember‑me” preference.

Key Protocols, Ports and Their Purposes

Protocol	Purpose	Default Port
DNS	Translate domain names to IP addresses	53 (UDP/TCP)
TCP	Reliable, ordered data transport	Varies (e.g., 80, 443)
HTTP	Transfer web resources (unencrypted)	80
HTTPS (HTTP over TLS)	Secure transfer – encryption, authentication, integrity	443

Suggested diagram: Flowchart showing the sequence – URL entry → DNS lookup → TCP/TLS handshake → HTTP/HTTPS request → Server response → Rendering.

Summary

Entering a URL triggers a chain of operations that converts a human‑readable address into a machine‑readable IP address, establishes a reliable (and possibly encrypted) connection, requests the required resource using HTTP or HTTPS, and finally renders the page for the user. Understanding each step helps learners see how the Internet delivers web pages instantly across the globe.

Describe how web pages are located, retrieved and displayed on a device when a user enters a URL