Unit 11: Internet

DECAP145 7 min read

The Internet is a global network of interconnected computer networks that communicate using standardised protocols (chiefly TCP/IP), enabling billions of devices to exchange data. This unit surveys the vocabulary of the web, the languages that build web content, the divide between client-side and server-side scripting, and the major applications that run on top of the network.

I. Orientation: The Network and Its Vocabulary

The Internet (evolved from ARPANET, 1969) is the physical and logical infrastructure; the World Wide Web (proposed by Tim Berners-Lee, 1989) is one service running on it. Distinguishing the two is the foundation for everything that follows.

Internet vs Web: the Internet is the hardware and protocol layer (routers, cables, TCP/IP); the Web is a collection of interlinked documents served over HTTP.
Client-server model: most Internet interaction pairs a client (requests a resource) with a server (supplies it).
Protocols: agreed rules for communication — HTTP/HTTPS for web pages, SMTP for mail, FTP for files.
Addressing: every host has a numeric IP address; DNS translates human-readable domain names into those addresses.

II. Basic Internet Terms

A. Definition and role

These terms name the building blocks a user meets when browsing.

Web Page: a single document written in HTML, viewable in a browser; may contain text, images, links and media. Has its own file such as about.html.
Website: a collection of related web pages under one domain name, for example www.example.com, stored together on a web server.
Homepage: the default or introductory page of a website, loaded first when the domain is entered; acts as the entry point and navigation hub.
Browser: a software application that requests, interprets and displays web pages (Chrome, Firefox, Edge, Safari). It renders HTML, runs scripts and manages the connection.
URL (Uniform Resource Locator): the complete address of a resource on the web.

TEXT

https://www.example.com:443/products/item.html?id=5#top

Scheme: https — protocol used.
Host: www.example.com — domain name.
Port: 443 — service port (optional).
Path: /products/item.html — file location.
Query: ?id=5 — parameters; Fragment: #top — page anchor.

Hypertext: text containing links (hyperlinks) that jump to other documents or positions; clicking a link navigates non-sequentially. It is the concept HTML implements.
ISP (Internet Service Provider): a company that provides users access to the Internet (via broadband, fibre, mobile data), assigns IP addresses and routes traffic.
Web Server: a computer plus software (Apache, Nginx, IIS) that stores website files and responds to HTTP requests by sending pages back to browsers.

III. Markup and Content Languages

A. Purpose and principle

Markup languages describe the structure and content of documents using tags; the browser interprets these tags to render the page.

B. HTML (HyperText Markup Language)

HTML is the standard language for creating web page structure and content.

Structure: uses paired tags enclosing content, organised in a nested tree.
Static nature: describes structure only; the page does not change after loading without scripting.
Example:

HTML

<!DOCTYPE html>
<html>
  <head><title>My Page</title></head>
  <body>
    <h1>Welcome</h1>
    <p>This is a <a href="page2.html">link</a>.</p>
  </body>
</html>

<head>: metadata such as the title.
<body>: visible content — headings, paragraphs, links.

C. DHTML (Dynamic HTML)

DHTML is not a language but a combination of technologies that make pages interactive and changeable after loading.

Components: HTML (structure) + CSS (presentation) + JavaScript (behaviour) + the DOM (Document Object Model).
Capability: lets content change in response to user action without reloading — e.g. a menu expanding on hover, text changing colour on click.
DOM role: exposes page elements as objects the script can read and modify.

D. XML (eXtensible Markup Language)

XML is a language for storing and transporting structured data, using tags the author defines.

Extensible: unlike HTML's fixed tag set, users create their own tags to describe data meaning.
Data, not display: XML carries data; it does not specify how content looks.
Well-formed rules: every tag must be closed and properly nested; one root element required.
Example:

XML

<book>
  <title>Networking Basics</title>
  <author>R. Sharma</author>
  <price>350</price>
</book>

Contrast with HTML: HTML tags like <p> are predefined; XML tags like <price> are invented to describe the data.

IV. Client-Side and Server-Side Scripting

A. Definition and principle

Scripting adds dynamic behaviour and logic to web pages; where the script executes divides the two categories.

B. Client-side scripting

Runs inside the user's browser after the page is downloaded.

Execution: the browser interprets the code locally; no server round-trip needed.
Languages: JavaScript (chief example), formerly VBScript.
Uses: form validation, animations, responding to clicks, updating parts of the page.
Trait: fast response, reduces server load, but source is visible to the user and depends on browser support.

C. Server-side scripting

Runs on the web server before the page is sent to the browser.

Execution: server processes the script, builds an HTML page, then sends only the result to the client.
Languages: PHP, ASP.NET, JSP, Python, Node.js.
Uses: database access, user authentication, processing form data, generating personalised pages.
Trait: code is hidden from the user, can access files and databases, but adds server workload.

Contrast:

Client-side: executes in browser, handles presentation and quick interaction, code exposed.
Server-side: executes on server, handles data and business logic, code secure.

V. Internet Applications

A. Overview

These are the services users actually run over the Internet, each using its own protocols.

B. WWW (World Wide Web)

A system of interlinked hypertext documents accessed over the Internet via browsers.

Mechanism: browser sends an HTTP request for a URL; the web server returns HTML which the browser renders.
Key elements: URLs (addressing), HTTP (transfer), HTML (content), hyperlinks (navigation).
Scope: the Web is a subset of Internet services, not the Internet itself.

C. E-mail (Electronic Mail)

Exchange of digital messages between users through mail servers.

Protocols: SMTP sends mail; POP3 or IMAP retrieves it.
Address format: username@domain.com — mailbox name and mail server domain.
Features: attachments, CC/BCC, folders; asynchronous, so the recipient need not be online.

D. Instant Messaging (IM)

Real-time text communication between users who are online simultaneously.

Nature: near-instant delivery, presence indicators (online/away/typing).
Examples: WhatsApp, Messenger, Slack.
Contrast with e-mail: synchronous and conversational rather than store-and-forward.

E. Internet Telephony (VoIP)

Voice calls carried over the Internet instead of traditional phone lines.

Technology: VoIP (Voice over Internet Protocol) digitises voice into data packets and transmits over IP.
Examples: Skype voice calls, WhatsApp calls.
Advantage: low cost, especially for long-distance, using existing Internet connection.

F. Video Conferencing

Real-time transmission of both audio and video among two or more participants.

Requirements: camera, microphone, speakers, and sufficient bandwidth.
Uses: remote meetings, online classes, telemedicine.
Examples: Zoom, Google Meet, Microsoft Teams; supports screen sharing and group calls.

G. Web Browser and its environment

The browser is the client application that presents web content and hosts the user's browsing session.

Rendering engine: parses HTML and CSS and paints the page on screen (e.g. Blink in Chrome).
JavaScript engine: executes client-side scripts (e.g. V8).
Environment components:
- Address bar: where the user types the URL.
- Tabs: allow multiple pages open in one window.
- Bookmarks/Favourites: save frequently visited URLs.
- History: record of visited pages.
- Cache: stores copies of files locally to speed reloading.
- Cookies: small data files a site stores in the browser to remember state such as login.
- Plugins/extensions: add extra features like ad-blocking or PDF viewing.
Navigation controls: back, forward, refresh and stop buttons manage page loading.

VI. How the Pieces Fit Together

A single browsing action ties the whole unit together and shows the client-server model in motion.

Step 1 — request: the user types a URL; the browser (client) asks the ISP to route the request to the correct web server via DNS lookup.
Step 2 — processing: the web server may run server-side scripts to build the page, drawing on a database if needed.
Step 3 — response: the server returns HTML (possibly with CSS, XML data and JavaScript) over HTTP.
Step 4 — rendering: the browser's engine displays the page; client-side scripts and DHTML then handle interaction locally.
Beyond the Web: the same network simultaneously carries e-mail (SMTP), instant messages, VoIP calls and video conferencing, each using its own protocol over the shared Internet infrastructure.