Explain the purpose and functions of a web browser

Cambridge IGCSE Computer Science (0478) – The Internet and its Uses: Web Browser

Objective

Explain the purpose and functions of a web browser, including URL structure, the HTTP/HTTPS request‑response cycle, DNS resolution, caching, cookies, and basic security features. Relate each point to the relevant IGCSE assessment objectives (AO1–AO3).

1. The Internet vs. the World‑Wide‑Web

• Internet: A global network of computers, routers and other devices that exchange data using the TCP/IP protocol suite.
• World‑Wide‑Web (WWW): One of many services that run on the Internet. It uses the Hypertext Transfer Protocol (HTTP/HTTPS) to request and deliver hypertext documents (web pages).
• Domain Name System (DNS): A distributed directory that translates a human‑readable domain name (e.g., www.example.com) into a numeric IP address (e.g., 93.184.216.34) that computers use to locate each other on the Internet.

2. What Is a Web Browser?

A web browser is a software application that acts as the user’s gateway to the WWW. It:

• Accepts a web address (URL) from the user.
• Resolves the URL to an IP address via DNS.
• Communicates with web servers using HTTP or HTTPS.
• Retrieves, interprets and displays the requested resources (HTML, CSS, JavaScript, images, etc.).
• Provides tools for navigation, data storage (caches, cookies), and security.

3. URL Structure

A Uniform Resource Locator (URL) tells the browser where a resource is located and how to obtain it.

Full example: https://www.example.com/articles/index.html?id=25&type=article

4. Core Purposes of a Web Browser

• Access and render web pages.
• Navigate between resources using hyperlinks.
• Interact with web applications (forms, online games, e‑learning tools).
• Store temporary data (caches, cookies) to speed up loading and maintain session state.
• Provide security measures such as SSL/TLS validation, safe‑browsing warnings and sandboxing.

5. Main Functions of a Web Browser

6. HTTP vs. HTTPS – Request/Response Cycle

1. Browser opens a TCP connection to the server’s IP address (port 80 for HTTP, port 443 for HTTPS).
2. HTTPS only: SSL/TLS handshake
   • Server sends its digital certificate.
   • Browser verifies the certificate against trusted Certificate Authorities.
   • Both sides agree on a symmetric encryption key.
3. Browser sends an HTTP request line (e.g., GET /index.html HTTP/1.1) followed by header fields such as Host, User‑Agent, Accept, and Cookie.
4. Server replies with a status line (e.g., HTTP/1.1 200 OK or HTTP/1.1 404 Not Found) and response headers (Content‑Type, Cache‑Control, Set‑Cookie, etc.).
5. If the status is 200, the body contains the requested HTML, CSS, JavaScript, images, …
6. The browser processes the response using the functions listed in Section 5.

7. Step‑by‑Step: How a Browser Retrieves a Web Page

8. Cookies – Session vs. Persistent

• Session cookie: Stored only in memory; deleted when the browser is closed. Typical use – shopping‑cart identifier.
• Persistent cookie: Saved to disk with an expiry date; survives browser restarts. Typical use – “remember me” login or site preferences.

Example of a Set‑Cookie header sent by a server:

Set-Cookie: userId=abc123; Expires=Wed, 10 Jan 2027 12:00:00 GMT; Path=/; Secure; HttpOnly

Trace‑table practice (exam‑style):

When the user revisits the site, the browser automatically includes the stored cookies in the Cookie request header, e.g.:

Cookie: session=xyz; theme=dark

9. Caching – Why It Matters (and a Quick Flowchart)

• Reduces bandwidth consumption.
• Speeds up page loading by re‑using previously downloaded resources.
• Relies on HTTP headers such as Cache‑Control, Expires and ETag to decide freshness.

10. Security Features (linked to AO3)

• SSL/TLS validation: The browser checks the server’s certificate against a trusted list of Certificate Authorities. AO3 relevance: Students should evaluate why encrypted connections protect data confidentiality and integrity.
• Safe‑browsing warnings: Blacklists and heuristic analysis block known phishing or malware sites. AO3 relevance: Explain why such warnings reduce the risk of credential theft.
• Sandboxing: Each tab runs in an isolated process, preventing malicious script from accessing the local file system or other tabs. AO3 relevance: Justify sandboxing as a defence‑in‑depth measure.
• Mixed‑content blocking: Modern browsers refuse to load insecure (HTTP) resources on an HTTPS page, protecting against downgrade attacks.

11. Common Browser Features

• Tabs: Multiple pages displayed within a single window.
• Incognito / Private mode: No history, cookies or cached files are retained after the session ends.
• Extensions / Add‑ons: Optional tools such as ad‑blockers, password managers or language translators.
• Developer Tools: Inspect HTML/CSS, debug JavaScript, monitor network activity and view performance metrics.

Sidebar: Digital Currency (≈150 words)

Digital currency, also known as cryptocurrency, is a form of electronic money that uses cryptographic techniques to secure transactions and control the creation of new units. The most widely known example is Bitcoin, which operates on a public ledger called a blockchain. A blockchain is a chain of blocks, each containing a list of transactions; once a block is added, it cannot be altered without changing every subsequent block, making fraud extremely difficult.

In the context of the Internet, digital currencies enable peer‑to‑peer payments without a central bank or intermediary. They rely on public‑key cryptography: a user’s “public key” (the address) is shared openly, while the “private key” (known only to the owner) signs transactions to prove ownership. Although not part of the core web‑browser syllabus, understanding digital currency helps learners appreciate emerging web‑based services such as online wallets, smart contracts and decentralized applications (dApps).

Summary

A web browser is the gateway to the World‑Wide‑Web. It resolves URLs to IP addresses via DNS, establishes TCP or TLS connections, sends HTTP/HTTPS requests, interprets status codes, manages caches and cookies, renders HTML/CSS/JavaScript, and enforces security through SSL/TLS validation, safe‑browsing warnings and sandboxing. Mastery of these concepts satisfies the Cambridge IGCSE Computer Science requirements for explaining how information travels from a remote server to a user’s screen and for evaluating the security measures that protect that process.