Introduction to Computer Networks

Overview

Learning outcomes

• Students will be able to define a computer network and classify networks as PAN, LAN, MAN or WAN with examples.
• Students will be able to explain the functions of modem, hub, switch, repeater, router and gateway and compare hub, switch and router.
• Students will be able to describe star, bus, tree and mesh topologies with their advantages and disadvantages.
• Students will be able to differentiate the Internet from the WWW and identify the parts of a URL.
• Students will be able to explain email, chat and VoIP, and differentiate a website from a web page and a static page from a dynamic page.
• Students will be able to explain the role of a web server and the steps to host a website.
• Students will be able to configure browser settings and explain add-ons, plug-ins and cookies.
• Students will be able to solve campus-network case studies: cable layout, server placement, devices and topology.

Lesson plan

Notes

Star                          Bus
      [PC1]
        |                [PC1]   [PC2]   [PC3]   [PC4]
[PC4]--[SWITCH]--[PC2]     |       |       |       |
        |               T==+=======+=======+=======+==T
      [PC3]                    backbone (T = terminator)

Key terms

Computer network

Two or more devices connected by a wired or wireless medium to share data and resources.

Node

Any device connected to a network, such as a computer, printer or phone.

PAN

Personal Area Network: devices of one person within about 10 metres, e.g. phone and earbuds.

LAN

Local Area Network: a network within a building or campus, up to about 1 km.

MAN

Metropolitan Area Network: a network covering a city, about 30-40 km.

WAN

Wide Area Network: a network across countries or continents; the Internet is the largest WAN.

Modem

Device that converts digital signals to analog and back so data can travel over telephone or cable lines.

Repeater

Device that regenerates a weakened signal so it can travel a longer distance.

Hub

Device that connects LAN nodes and broadcasts incoming data to all ports.

Switch

Device that connects LAN nodes and sends data only to the destination port using MAC addresses.

Router

Device that connects different networks and forwards packets by IP address along the best path.

Gateway

Entry and exit point of a network; can connect networks that use different protocols.

Topology

The physical or logical arrangement of nodes and links in a network.

MAC address

Unique 48-bit hardware address of a network interface card.

IP address

Numeric logical address that identifies a device on the Internet.

URL

Uniform Resource Locator: the complete address of a resource on the Internet.

DNS

Domain Name System: converts domain names into IP addresses.

VoIP

Voice over Internet Protocol: making voice calls over the Internet.

Web server

Computer and software that store websites and send web pages to browsers on request.

Cookie

Small text file stored by a website in the browser to remember information about the user.

MCQs

Tap an option to check it, or use Show answers above to reveal them all.

1. Bluetooth connection between a phone and a smartwatch is an example of a:

   1. LAN
   2. PAN
   3. MAN
   4. WAN
   Answer

   (b) PAN

   A PAN connects the personal devices of one person within about 10 m.

2. The network connecting all computers in a school's three-storey building is a:

   1. LAN
   2. PAN
   3. MAN
   4. WAN
   Answer

   (a) LAN

   A single building or campus forms a LAN.

3. Which is the largest WAN in the world?

   1. ARPANET
   2. Intranet
   3. The Internet
   4. NICNET
   Answer

   (c) The Internet

   The Internet is a worldwide network of networks.

4. Which device converts digital signals to analog signals and back?

   1. Repeater
   2. Switch
   3. Gateway
   4. Modem
   Answer

   (d) Modem

   MOdulator-DEModulator: modulation turns digital to analog, demodulation turns analog to digital.

5. Which device sends incoming data to all of its ports?

   1. Hub
   2. Switch
   3. Router
   4. Modem
   Answer

   (a) Hub

   A hub broadcasts data to every connected node.

6. A switch forwards data to the correct computer using its:

   1. IP address
   2. URL
   3. MAC address
   4. Domain name
   Answer

   (c) MAC address

   A switch keeps a table of MAC addresses of the device on each port.

7. Which device should be used to connect a school LAN to the Internet?

   1. Hub
   2. Repeater
   3. Switch
   4. Router
   Answer

   (d) Router

   A router connects different networks and forwards packets between them using IP addresses.

8. Two buildings of a factory are 180 m apart and joined by an Ethernet cable. Which device is needed to keep the signal strong?

   1. Gateway
   2. Repeater
   3. Modem
   4. Hub
   Answer

   (b) Repeater

   Twisted-pair Ethernet carries signals reliably for about 100 m; a repeater regenerates the signal over longer runs.

9. A device that acts as the entry and exit point of a network and can join networks using different protocols is a:

   1. Switch
   2. Hub
   3. Gateway
   4. Repeater
   Answer

   (c) Gateway

   A gateway is the node through which traffic enters or leaves a network.

10. In which topology does failure of the central device stop the whole network?

    1. Star
    2. Mesh
    3. Bus
    4. None of these
    Answer

    (a) Star

    In star topology all traffic passes through the central switch or hub.

11. Which topology uses a single backbone cable with terminators at both ends?

    1. Star
    2. Tree
    3. Mesh
    4. Bus
    Answer

    (d) Bus

    In bus topology all nodes share one backbone cable.

12. How many links are needed to connect 6 computers in a full mesh?

    1. 6
    2. 12
    3. 15
    4. 30
    Answer

    (c) 15

    n(n-1)/2 = 6 x 5 / 2 = 15.

13. Several star networks connected to a common backbone form a:

    1. tree topology
    2. bus topology
    3. mesh topology
    4. ring topology
    Answer

    (a) tree topology

    Tree is a hierarchical hybrid of star and bus.

14. In the URL https://www.ncert.nic.in/textbook.php, the domain name is:

    1. https
    2. www.ncert.nic.in
    3. textbook.php
    4. nic
    Answer

    (b) www.ncert.nic.in

    The domain (host) name comes after the protocol and before the path.

15. Who invented the World Wide Web?

    1. Vint Cerf
    2. Bill Gates
    3. Ray Tomlinson
    4. Tim Berners-Lee
    Answer

    (d) Tim Berners-Lee

    Tim Berners-Lee developed the WWW at CERN in 1989-90.

16. Making a voice call through WhatsApp over mobile data uses:

    1. VoIP
    2. FTP
    3. SMTP
    4. DNS
    Answer

    (a) VoIP

    VoIP carries voice as data packets over the Internet.

17. A page that shows a student's board result after the roll number is entered is a:

    1. static web page
    2. dynamic web page
    3. home page only
    4. plug-in
    Answer

    (b) dynamic web page

    The content is generated from a database for each request, so it is dynamic.

18. Which of these is web server software?

    1. Google Chrome
    2. MySQL
    3. Apache
    4. Notepad
    Answer

    (c) Apache

    Apache HTTP Server delivers web pages. Chrome is a browser; MySQL is a database.

19. A small text file stored by a website in your browser to remember your login or preferences is a:

    1. plug-in
    2. cookie
    3. cache virus
    4. bookmark
    Answer

    (b) cookie

    Cookies store small pieces of information about the user's visit.

20. Which statement about browser add-ons is correct?

    1. Extensions can only play videos
    2. Plug-ins are web servers
    3. Add-ons are needed to open any web page
    4. An extension adds features to the browser, such as an ad blocker
    Answer

    (d) An extension adds features to the browser, such as an ad blocker

    Extensions add features to the browser; plug-ins help it display particular content.

Assertion–Reason

1. Assertion (A): A switch is preferred over a hub in a modern LAN.

   Reason (R): A switch sends data only to the port of the destination device, reducing traffic and collisions.

   Answer

   (A)

   Both are true and R is the reason switches are preferred.

2. Assertion (A): In bus topology, a break in the backbone cable can stop the entire network.

   Reason (R): Bus topology needs less cable than star topology.

   Answer

   (B)

   Both are true, but needing less cable does not explain why a backbone break stops the network.

3. Assertion (A): The Internet and the World Wide Web mean the same thing.

   Reason (R): The World Wide Web is a service that runs on the Internet.

   Answer

   (D)

   A is false; R is true and in fact shows why they are different.

4. Assertion (A): A mesh topology is highly reliable.

   Reason (R): In a mesh topology, there are multiple paths between nodes, so data can take another path if a link fails.

   Answer

   (A)

   Both are true and R explains A.

5. Assertion (A): A cookie is a small text file stored by a website on the user's computer.

   Reason (R): Cookies are programs that infect and damage the files on a computer.

   Answer

   (C)

   A is true. R is false: cookies are data, not programs, though they can raise privacy concerns.

Short answer

1. 2 marks

   What is a computer network? Give two advantages of networking the computers of a school.

   Answer

   A computer network is a group of two or more devices connected by a wired or wireless medium to share data and resources.

   Advantages: (1) resources such as one printer and one Internet connection can be shared by the whole lab; (2) data such as student records can be stored centrally and shared between the office and teachers. (Also: communication by email/chat, lower cost.)

2. 2 marks

   Differentiate between LAN and WAN (any two points).

   Answer

   • Area: a LAN covers a building or campus (up to about 1 km); a WAN spreads across cities, countries or continents.
   • Speed and ownership: a LAN is faster and usually owned by one organisation; a WAN is slower, costlier and uses lines owned by telecom companies. Example: school lab (LAN), the Internet (WAN).
3. 2 marks

   What is a MAN? Give one example.

   Answer

   A Metropolitan Area Network covers a city or town (about 30-40 km) by connecting many LANs. Example: a cable TV network in a city, or a network linking all branches of a bank in one city.

4. 2 marks

   What is the function of a modem? Expand the term.

   Answer

   Modem stands for MOdulator-DEModulator. It converts digital data from a computer into analog signals for transmission over telephone or cable lines (modulation), and converts the received analog signals back into digital data (demodulation). It connects a user to the ISP.

5. 2 marks

   Differentiate between a hub and a switch.

   Answer

   • A hub broadcasts incoming data to all ports; a switch sends data only to the port of the destination device.
   • A hub does not use addresses and causes more traffic and collisions; a switch uses MAC addresses, so it is faster and more secure.
6. 2 marks

   What is a repeater? When is it required?

   Answer

   A repeater is a device that receives a weakened (attenuated) signal, regenerates it to its original strength and retransmits it. It is required when the cable between two devices is longer than the distance the signal can travel reliably, about 100 m for Ethernet twisted-pair cable.

7. 2 marks

   What is a gateway? How is it different from a router?

   Answer

   A gateway is the entry and exit point of a network through which all traffic to and from outside networks passes; it can connect networks that use different protocols. A router connects networks and chooses the best path for packets using IP addresses; a router often acts as the gateway, but a gateway's defining role is being the network's doorway and translating between dissimilar networks.

8. 2 marks

   Give one advantage and one disadvantage each of star and bus topologies.

   Answer

   • Star: advantage: failure of one node or cable does not affect others and faults are easy to find. Disadvantage: if the central switch fails, the whole network stops.
   • Bus: advantage: needs the least cable and is cheap. Disadvantage: a break in the backbone stops the whole network, and faults are hard to locate.
9. 2 marks

   Identify the parts of the URL https://www.indianrail.gov.in/enquiry/pnr.html.

   Answer

   • Protocol: https
   • Domain name: www.indianrail.gov.in (top-level domain .in)
   • Path: /enquiry/
   • File name: pnr.html
10. 2 marks

    Differentiate between a website and a web page.

    Answer

    A web page is a single document on the Web, written in HTML and identified by its own URL. A website is a collection of related, interlinked web pages under one domain name, stored on a web server, with a home page as its starting point. Example: the CBSE website contains many web pages such as the circulars page and the results page.

11. 2 marks

    Differentiate between static and dynamic web pages, with one example each.

    Answer

    • A static page has fixed content that is the same for every visitor; it is written in HTML/CSS and changes only when edited. Example: a school's 'About Us' page.
    • A dynamic page's content is generated when requested, often from a database, and can differ by user or input; it uses server-side scripts such as PHP or JSP. Example: an online result page or a shopping cart.
12. 2 marks

    What are cookies? Give one use and one risk.

    Answer

    Cookies are small text files that a website stores in the user's browser to remember information about the visit. Use: keeping a user logged in, or remembering items in a shopping cart. Risk: third-party cookies can track a user across websites and invade privacy; a stolen session cookie can be used to take over a logged-in account.

13. 2 marks

    What is VoIP? Give one advantage and one disadvantage.

    Answer

    VoIP (Voice over Internet Protocol) is the technology of making voice calls over the Internet by converting voice into data packets. Advantage: calls are very cheap or free, even international ones. Disadvantage: call quality depends on the Internet connection, and calls fail during network or power outages.

Long answer

1. 3 marks

   Explain any three network devices: modem, switch and router. Mention where each is used in a school.

   Answer

   • Modem: converts digital signals to analog for transmission over telephone or cable lines and back to digital at the other end. In a school, it connects the school network to the ISP's broadband line (often built into the broadband router).
   • Switch: connects the computers of a LAN and forwards each packet only to the destination port using MAC addresses, reducing traffic. In a school, each computer lab and the office has a switch to which all computers are connected.
   • Router: connects different networks and forwards packets using IP addresses along the best route. In a school, it connects the school LAN to the Internet and can separate the student lab network from the office network.
2. 3 marks

   What is a network topology? Explain tree and mesh topologies with one advantage and one disadvantage each.

   Answer

   A topology is the arrangement (layout) of nodes and connecting links in a network.

   • Tree: star networks are connected to a common backbone or a root switch in a hierarchy. Advantage: suits large networks spread over floors or buildings and is easy to expand branch by branch. Disadvantage: if the backbone or root device fails, branches lose contact with each other.
   • Mesh: each node is connected to every other node (or to many nodes), giving multiple paths. Advantage: very reliable, as data takes another path if a link fails. Disadvantage: needs a large amount of cabling and ports, so it is costly and hard to install.
3. 3 marks

   What is the World Wide Web? How is it different from the Internet? Name the three technologies on which the Web is built.

   Answer

   The World Wide Web is a collection of interlinked web pages and resources, stored on web servers and accessed through the Internet using a browser. It was developed by Tim Berners-Lee in 1989-90.

   The Internet is the global network of networks (the infrastructure), while the Web is one service that runs on it; email, chat, VoIP and file transfer also use the Internet but are not the Web.

   The three technologies: HTML (language of web pages), URL/URI (unique address of each resource) and HTTP (protocol for requesting and sending pages).

4. 3 marks

   What is a web server? List the steps to host a website for a school named 'Green Valley Public School'.

   Answer

   A web server is a computer with server software (such as Apache or Nginx) that stores websites and sends web pages to browsers when they request them using HTTP. It stays connected to the Internet all the time.

   Steps to host the school's website:

   1. Choose and register an available domain name, e.g. greenvalleyps.in, with a domain registrar.
   2. Select a web hosting provider and a suitable plan (shared hosting is enough for a school) and get space on its web server.
   3. Upload the website's files (HTML pages, images, scripts, database) to the server through the control panel or FTP.
   4. Point the domain name to the server's IP address using DNS records, then test the site in a browser.
5. 5 marks

   What is a web browser? Explain any four browser settings that a student should know, and the difference between add-ons/extensions and plug-ins.

   Answer

   A web browser is application software that requests web pages from web servers and displays them, letting the user navigate using hyperlinks. Examples: Chrome, Firefox, Edge, Safari.

   Settings:

   1. Home page/startup: the page that opens first.
   2. Default search engine: used when searching from the address bar.
   3. Privacy and security: clearing history, cache and cookies, blocking third-party cookies, Safe Browsing.
   4. Site permissions: allowing or blocking camera, microphone, location, pop-ups and notifications for each site.

   (Also: saved passwords/autofill, download location, private browsing.)

   An extension (add-on) adds a feature to the browser itself, such as an ad blocker or password manager. A plug-in lets the browser display a particular type of content inside a page that it cannot handle on its own, such as an older video or document format.

6. 5 marks

   Explain the Internet applications email, chat and VoIP. Compare email and chat on any two points.

   Answer

   • Email: sending messages with attachments to one or more people using addresses of the form user@domain. Messages wait on the mail server until read. Example: Gmail.
   • Chat: real-time exchange of text (and images, voice notes) between people who are online at the same time. Example: WhatsApp.
   • VoIP: voice (and video) calls over the Internet; the voice is sent as data packets. Cheap, but depends on Internet quality. Example: Google Meet, WhatsApp call.

   Email vs chat: (1) email is asynchronous, so the receiver can read it later, while chat is real-time; (2) email suits formal and longer messages with attachments and subject lines, while chat suits short, informal, quick messages.

7. 3 marks

   Compare PAN, LAN, MAN and WAN on area, speed, cost and example.

   Answer

   • PAN: about 10 m around one person; low to moderate speed; very low cost; e.g. phone connected to earbuds by Bluetooth.
   • LAN: a building or campus up to about 1 km; very high speed; low cost; e.g. school computer lab.
   • MAN: a city, about 30-40 km; high speed; high cost; e.g. a city's cable TV network.
   • WAN: countries and continents; lower and variable speed; very high cost; e.g. the Internet, railway reservation network.

Case-based

Practicals

Model answer (sample school):
- Computer lab: 30 PCs, each with an Ethernet cable to one 48-port switch.
- The lab switch connects to the main switch in the server room, which also connects the
  office switch (8 PCs, 1 printer) and the library switch (4 PCs).
- The main switch connects to the router, which connects to the ISP's fibre line (ONT/modem).
- Wi-Fi access points in the staff room connect to the main switch.

Topology: each room is a STAR (PCs around a switch); the room switches hang from the
main switch, so the whole campus is a TREE topology.
Network type: LAN (single campus); the connection through the router to the Internet links it to a WAN.
Devices identified: switch, router, modem/ONT, wireless access point, NIC in each PC.

Server: OPD block (maximum computers, 60).

Cable layout (least total cable, every link under 100 m):
    OPD --50 m-- Labs --70 m-- Wards --40 m-- Pharmacy
Total = 160 m. (If the Wards should be one hop from the server, use
OPD-Labs 50 m, OPD-Wards 90 m, Wards-Pharmacy 40 m = 180 m; still no link over 100 m.)

Topology: computers in each block connect to a switch (star); blocks connect in a
hierarchy from OPD, so the campus is a TREE topology.

Devices: one switch in each block; a router (gateway) in OPD to connect to the ISP;
a firewall at the gateway for security.

Repeater: not needed in this layout (all links < 100 m). If Pharmacy were joined
directly to OPD (130 m) or Labs (110 m), a repeater would be required on that link.

Nagpur-Nashik link: a WAN over a leased line or fibre broadband from an ISP, through
routers at both ends (VPN over the Internet is an economical option).

Model answer:
1. https://www.cbse.gov.in/cbsenew/cbse.html
   protocol=https, domain=www.cbse.gov.in, TLD=.in, path=/cbsenew/, file=cbse.html
2. https://ncert.nic.in/textbook.php
   protocol=https, domain=ncert.nic.in, TLD=.in, path=/, file=textbook.php
3. https://diksha.gov.in/explore
   protocol=https, domain=diksha.gov.in, TLD=.in, path=/explore (a page route, no file name)
4. https://www.irctc.co.in/nget/train-search
   protocol=https, domain=www.irctc.co.in, TLD=.in, path=/nget/train-search
5. https://en.wikipedia.org/wiki/Computer_network
   protocol=https, domain=en.wikipedia.org, TLD=.org, path=/wiki/, resource=Computer_network

Why https: data between the browser and the server is encrypted, so passwords and
payment details cannot be read by others on the network; the padlock icon also
confirms the site's identity through its security certificate.

Model record (Google Chrome; other browsers are similar):
(a) Settings > On startup > Open a specific page > Add a new page > school URL.
(b) Settings > Search engine > choose a different engine from the list.
(c) Settings > Privacy and security > Third-party cookies > See all site data and
    permissions > search the site > Delete. Observation: the site asked us to log in again.
(d) Settings > Privacy and security > Third-party cookies > Block third-party cookies.
(e) Settings > Privacy and security > Site settings > Camera / Microphone: sites that
    asked permission are listed under Allowed / Not allowed.
(f) Menu > Extensions > Manage Extensions: e.g. a PDF tool and an ad blocker were installed.

Observation: after deleting a site's cookies, its saved login and preferences were
lost, which shows that cookies are what let a site remember the user.

Static page saved as myclass.html:

<html>
  <head><title>Class XII-B</title></head>
  <body>
    <h1>Class XII-B, Sunrise School</h1>
    <p>Class teacher: Mrs. Kaur. Students: 38.</p>
    <a href="https://ncert.nic.in">NCERT textbooks</a>
  </body>
</html>

Comparison:
- myclass.html shows exactly the same content to every user until the file is edited.
  It needs no server-side program or database.
- A train-search page shows different trains and seats for each user's stations and
  date, generated by a server-side program reading a database at that moment.
  So it is dynamic.

Common mistakes

• Saying the Internet and the WWW are the same. The Internet is the global network; the WWW is a collection of linked web pages that runs on it, alongside email, chat and VoIP.

• Writing that a hub sends data only to the destination. That is a switch; a hub broadcasts to all ports.

• Confusing a router with a switch. A switch connects devices within a LAN using MAC addresses; a router connects different networks using IP addresses.

• Placing the server in the block nearest to the main gate or the head office in a case study. The server goes in the block with the maximum number of computers (most traffic stays local).

• Forgetting the repeater in a campus case study. Any cable run longer than about 100 m needs a repeater.

• Calling every page with images or animation 'dynamic'. A page is dynamic only when its content is generated on request (often from a database); a page with fixed images and animations is still static.

• Thinking incognito or private mode makes a user anonymous. It only stops the browser from keeping history and cookies on that device; websites, the school network and the ISP can still see the activity.

• Calling a cookie a virus or a program. A cookie is only a small text file; the concern is privacy and tracking, not infection.