Computer Networks & Internet Essentials

Introduction to Computer Networks

  • Network: interconnection of \ge 2 similar entities (people, devices) for the purpose of communication and resource sharing. Provides a foundation for collaboration and information exchange.

  • Computer network: connects diverse computing devices (servers, PCs, laptops, smartphones, IoT devices) to facilitate the sharing of data, hardware resources (e.g., printers, scanners), and applications across different locations.

  • Data sent as packets: Information is broken down into small, manageable units called packets, which are then transmitted across the network. This transmission occurs via various media, including wired (e.g., Ethernet cables, fibre optic cables) or wireless (e.g., airwaves for Wi-Fi, cellular networks) channels. Network protocols (e.g., TCP/IP) govern how these packets are formatted, addressed, transmitted, routed, and received.

Types of Networks

  • LAN (Local Area Network)

    • Range: Typically covers a confined geographical area, from a single room or building to a campus, extending up to approximately 1 kilometer (1000 meters).

    • Media: Primarily uses high-speed Ethernet cables (twisted pair, fibre optic) for wired connections and Wi-Fi (802.11 standards) for wireless access.

    • Speed: Data transfer rates range from 10 Mbps (original Ethernet) to 1000 Mbps (Gigabit Ethernet) and even 10 Gbps or 40 Gbps with newer standards. Often characterized by high bandwidth and low latency.

    • Characteristics: Highly secure due to limited physical access; offers very high data rates suitable for sharing files, printers, and internal applications in homes, offices, or educational institutions.

  • MAN (Metropolitan Area Network)

    • Covers a larger area than a LAN, encompassing a city or town, typically spanning circa 30–40 kilometers. Connects multiple LANs within a municipal boundary.

    • Speed: Generally offers lower speeds than a LAN but higher than a WAN. Examples include cable-TV networks, city-wide broadband internet services, or interconnected university campuses. Often uses technologies like Fibre Distributed Data Interface (FDDI) or Asynchronous Transfer Mode (ATM).

  • WAN (Wide Area Network)

    • Links LANs and MANs across vast geographical distances, spanning countries or even continents. The Internet is the largest and most prominent example of a WAN.

    • Characteristics: Characterized by lower speeds and higher latency compared to LANs and MANs due to the longer transmission distances and reliance on public telecommunication infrastructure. Technologies like MPLS (Multiprotocol Label Switching) and VPNs (Virtual Private Networks) are crucial for efficient and secure WAN communication.

Key Network Devices

  • Modem (MOdulator DEModulator): A device that converts digital signals from a computer into analog signals suitable for transmission over analog lines (e.g., telephone lines, cable lines) and vice versa. Essential for connecting to ISPs (Internet Service Providers) via DSL, cable, or fibre optic connections.

  • NIC / Ethernet Card (Network Interface Card): An expansion card or integrated circuit that enables a computer or device to connect to a network. It operates at the Data Link Layer (Layer 2) of the OSI model, providing a unique Media Access Control (MAC) address to identify the device on the network. Speeds typically range from 10 Mbps to 1 Gbps.

  • Repeater: A network device used to regenerate and retransmit weakened network signals over long cable runs (typically beyond ext{circa } 100 meters for Ethernet). It prevents signal degradation (attenuation) but does not filter or process data.

  • Hub: A basic network device that operates at the Physical Layer (Layer 1) of the OSI model. It acts as a central connection point, broadcasting any incoming data packet to all connected ports. This leads to frequent data collisions and inefficient network usage, especially in busy networks, as only one device can transmit at a time.

  • Switch: An intelligent network device that operates at the Data Link Layer (Layer 2). Unlike a hub, a switch learns the MAC addresses of devices connected to its ports and forwards incoming packets only to the specific destination port. This allows for simultaneous, dedicated communication paths, significantly reducing collisions and improving network performance. Can also manage VLANs (Virtual LANs) for network segmentation.

  • Router: A crucial inter-networking device that operates at the Network Layer (Layer 3). Routers analyze IP addresses to route data packets between different networks (e.g., your home LAN and the Internet). They maintain routing tables, can perform Network Address Translation (NAT), and often include firewall functionalities for security. Many home routers also combine modem and Wi-Fi access point functionalities.

  • Gateway: A network point that acts as an entry and exit point for data going from one network to another. Gateways can convert protocols between dissimilar networks, allowing communication between systems that use different communication architectures. For instance, a gateway might translate between TCP/IP and other legacy protocols, or facilitate communication between an internal network and external networks like the Internet.

Networking Topologies

  • Mesh: A topology where every node is directly connected to every other node in the network. The number of links required is calculated by the formula \frac{n(n-1)}{2}, where n is the number of nodes. It offers high reliability and fault tolerance due to multiple paths for data, but it is very costly and complex to implement for a large number of nodes.

  • Ring: Each node connects to exactly two other nodes, forming a single continuous pathway for signals in a circular fashion. Data typically flows unidirectionally (in one direction) around the ring. A break in one link can disrupt the entire network unless a dual ring (bidirectional) system is implemented.

  • Bus: All network nodes are connected to a common central backbone cable (bus). Data signals travel along this backbone. It is inexpensive and easy to install. However, it is susceptible to a single point of failure (a break in the backbone cable affects the entire network), and collisions can occur if multiple devices transmit simultaneously. Requires terminators at each end to absorb signals.

  • Star: All nodes are individually connected to a central hub, switch, or router. This is the most common topology in modern LANs. It is fast, easy to install, and simplifies fault isolation (a failure in one node's connection does not affect others). However, the entire network relies heavily on the central device; its failure will bring down the entire network.

  • Tree/Hybrid: A hierarchical arrangement that combines elements of bus and star topologies. Multiple star networks are connected to a central bus line, or a main bus branches out into secondary buses connected to star-configured nodes. This structure is highly scalable and fault-tolerant for individual branches, making it popular in larger corporate networks and WANs.

The Internet

  • Global WAN of computers, devices & network hardware: The Internet is a vast, interconnected global network that allows billions of devices worldwide to communicate. It is built upon the foundational TCP/IP (Transmission Control Protocol/Internet Protocol) suite, a set of communication protocols that define how data is transmitted over networks.

  • Built from ISPs → regional → national → global backbone: The Internet's structure is decentralized and tiered, starting from Internet Service Providers (ISPs) that provide access to end-users, connecting to regional networks, which in turn connect to national backbones, and finally to high-capacity global backbones that form the core of the Internet.

  • Ubiquitous uses: The Internet has permeated nearly every aspect of modern life, facilitating education (online learning), commerce (e-commerce, online banking), social media (communication and networking), entertainment (streaming, gaming), and the Internet of Things (IoT) for connecting smart devices.

Major Internet Services

  • WWW (World Wide Web): A global system of interlinked hypertext documents and other media accessible via the Internet. It is viewed through web browsers (clients) that request and render web content.

    • Core technologies: HTML (HyperText Markup Language) for structuring web content, URI/URL (Uniform Resource Identifier/Locator) for addressing and locating resources, and HTTP/HTTPS (Hypertext Transfer Protocol/Secure) for data transfer between web servers and browsers.

  • Email (Electronic Mail): A system for sending and receiving messages and attachments digitally over a network. It includes features like CC (Carbon Copy), BCC (Blind Carbon Copy), folders for organization, and spam filters to manage unsolicited messages. Key protocols include SMTP (Simple Mail Transfer Protocol) for sending and POP3 (Post Office Protocol version 3) or IMAP (Internet Message Access Protocol) for receiving.

  • Chat / Instant Messaging: Real-time text-based communication, often augmented with audio and video calls (e.g., WhatsApp, Skype, Zoom). These services enable immediate bidirectional communication between users, often leveraging underlying protocols like XMPP (Extensible Messaging and Presence Protocol) or proprietary communication protocols.

  • VoIP (Voice over IP): A technology that allows voice calls to be made over an Internet Protocol (IP) network instead of traditional telephone lines. It offers low cost, especially for long-distance and international calls, but call quality can depend heavily on available bandwidth and network stability. Uses protocols like SIP (Session Initiation Protocol) and RTP (Real-time Transport Protocol).

Website & Web Page

  • Website: A collection of related web pages, multimedia content, and other digital assets organized under a single domain name (e.g., \text{ncert.nic.in}). These are hosted on web servers and accessed via the Internet.

    • Purposes: Websites serve diverse functions, including selling products/services (e-commerce), providing information (news, educational), facilitating communication (forums, blogs), offering entertainment (video streaming, games), and distributing digital content.

  • Web page: A standalone document, typically written in HTML and formatted with CSS, that can be viewed in a web browser. A web page may include various forms of content such as text, images, audio, video, and interactive scripts.

    • Home page: The default or main page of a website, serving as the entry point or primary navigation hub.

    • Static page: Content remains identical for all users each time it is accessed. These pages are typically built using simple HTML and CSS, with any interactive elements handled by client-side JavaScript without server-side processing.

    • Dynamic page: Content is generated or customized in real-time based on user input, database queries, or other server-side logic. These pages require server-side processing using programming languages like PHP, JavaScript (Node.js), Python, Ruby, or Java, delivering personalized content to each user.

Web Server & Hosting

  • Web server (hardware/software): Refers to both the computer hardware that stores website files (HTML documents, images, videos, CSS stylesheets, JavaScript files) and the software (like Apache HTTP Server, Nginx, Microsoft IIS) that handles HTTP requests from web browsers. When a browser requests a page, the web server retrieves the relevant files and sends them back; if the page isn't found, it returns a 404 error.

  • Hosting steps:

    • Choose hosting provider & plan: Select a company that provides the necessary server space and internet connectivity. Considerations include resources (CPU, RAM, storage), database support, bandwidth allowance, backup policies, and security features (SSL certificates, firewalls). Hosting types include shared hosting, Virtual Private Servers (VPS), dedicated servers, and cloud hosting.

    • Register domain name: Acquire a unique web address (e.g., \text{example.com}) through a domain registrar. The Domain Name System (DNS) then maps this human-readable domain name to the numerical IP address of the web server where the website is stored.

    • Upload site files to server; configure logins, rights: Transfer all website files from your local computer to the web server, typically using FTP (File Transfer Protocol) or a control panel (e.g., cPanel). Configure database connections, user logins, and file permissions to ensure proper functionality and security.

Browser Basics

  • Browser (Chrome, Firefox, Edge, Safari, Opera): Application software used to access and view information on the World Wide Web. Browsers include a rendering engine (to display HTML/CSS), a JavaScript engine (to execute interactive scripts), and features for managing cookies, handling encryption (HTTPS), and supporting multimedia via plug-ins or built-in capabilities.

  • Settings panels (Firefox): Typically offer extensive customization options, including:

    • General: Basic display, language, startup behavior, and download preferences.

    • Home: Configure default web page displayed upon opening.

    • Search: Manage default search engines and search suggestions.

    • Privacy & Security: Control tracking protection, site permissions (camera, microphone), history, saved logins, and cookie management.

    • Sync: Synchronize bookmarks, history, passwords, and open tabs across multiple devices.

  • Add-ons/Extensions: Small software modules or programs that extend the functionality of a web browser (e.g., ad blockers, password managers, productivity tools). They are usually downloaded from an official browser store. Plug-ins: Separate software components required by the browser to display specific types of content, such as Flash Player for animations or Java Runtime Environment for Java applets (though largely deprecated now).

  • Cookies: Small text files that websites store on a user's computer via the web browser. They are used to remember user preferences, login status, shopping cart contents, or track browsing activity. Cookies can be session-based (temporary, deleted when browser closes) or persistent (stored for a set duration). Users can manage cookies via browser privacy settings, choosing to block, allow, or delete them, which is crucial for privacy control.