Computer Networks: CBSE Class 12 Computer Science Notes and Case Questions

What Computer Networks is about

Weightage: This chapter is part of Unit 2, Computer Networks, which carries 10 of the 70 theory marks together with Data Communication.Source: CBSE Computer Science (083) syllabus 2026-27, distribution of marks, Unit 2 Computer Networks

Key concepts

Computer network and the evolution of networking

NCERT section NCERT 10.1, 10.2

A computer network is a set of computers and other devices (nodes) connected by wired or wireless links so that they can share data, software and hardware such as printers, and communicate. Benefits: resource sharing, communication (e-mail, chat, video calls), reliability (data can be kept on more than one computer) and lower cost. ARPANET (Advanced Research Projects Agency Network) was a US Department of Defence project of the late 1960s that linked research computers; its first link joined UCLA and the Stanford Research Institute (1969). TCP/IP became its standard protocol in 1983. NSFNET, set up by the US National Science Foundation in 1986, built a faster backbone that connected universities and widened access. These networks and many others joined through TCP/IP to form the Internet, the global network of networks. The World Wide Web followed in 1990.

Worked example

Arrange in time order and state the role of each: Internet, ARPANET, NSFNET.

Try it first, then show the solutionSolution

1. ARPANET (1969): the first wide-area packet network, linking defence and research computers.
2. NSFNET (1986): a high-speed backbone that brought universities and research institutions on board.
3. Internet: the global network of networks formed as these and other networks interconnected using TCP/IP.

Answer: ARPANET → NSFNET → Internet.

Network devices

NCERT section NCERT 10.4

Device	Job
Modem	MOdulator-DEModulator: converts the computer's digital data to analog signals for a telephone or cable line and back
Ethernet card (NIC)	Network Interface Card: connects a computer to a wired network; each NIC has a unique MAC address
RJ45	8-pin connector at the ends of Ethernet (twisted pair) cables
Repeater	Regenerates a weakened signal so it can travel further (signals in Ethernet cable weaken after about 100 m)
Hub	Connects devices in a LAN and sends incoming data to all ports (broadcast); collisions are possible
Switch	Connects devices in a LAN but sends data only to the port of the destination device (uses its address table)
Router	Connects different networks and chooses the best path for each packet
Gateway	Connects networks that use different protocols, e.g. a LAN to the Internet; it is the entry and exit point
Wi-Fi card	Wireless network adapter that connects a device to a wireless LAN

Worked example

Name the device: (a) it connects a school LAN with the Internet, translating between different protocols (b) it forwards data only to the intended computer within a lab (c) it is placed midway in a 150 m cable run (d) it plugs into the end of an Ethernet cable.

Try it first, then show the solutionSolution

1. (a) Different protocols on the two sides: gateway (in practice a router often performs this role).
2. (b) Sending only to the destination port: switch (a hub would broadcast).
3. (c) Signals weaken over long cable: repeater.
4. (d) The 8-pin connector: RJ45.

Answer: (a) Gateway (b) Switch (c) Repeater (d) RJ45

Hub versus switch versus router

NCERT section NCERT 10.4.5-10.4.7

All three connect devices, but at different levels of intelligence. A hub is passive about addresses: whatever arrives on one port goes out on every other port, so bandwidth is shared and collisions happen. A switch learns which device is on which port and sends each frame only there, so it is faster and more secure inside a LAN. A router works between networks: it reads the destination IP address of each packet and forwards it towards the right network, choosing a route.

Worked example

A lab of 30 computers is slow because every computer receives all the traffic. Which device is probably in use, and what should replace it?

Try it first, then show the solutionSolution

1. Every computer receiving all traffic is the behaviour of a hub (broadcast).
2. A switch sends each frame only to the destination port, cutting unnecessary traffic and collisions.

Answer: A hub; replace it with a switch.

Types of network: PAN, LAN, MAN, WAN

NCERT section NCERT 10.3

Networks are classified by the area they cover:

Type	Area	Example
PAN (Personal Area Network)	about 10 m around one person	phone linked to earbuds and a laptop by Bluetooth or a hotspot
LAN (Local Area Network)	a room, building or campus, up to about 1 km	school computer lab, office network
MAN (Metropolitan Area Network)	a city, roughly 30-40 km	cable TV network of a city, links between a bank's branches in one city
WAN (Wide Area Network)	country, continent or the world	the Internet, a nationwide bank network

LANs are fast, cheap per node and privately owned; WANs are slower per link, cost more and usually use leased or public lines.

Worked example

Classify: (a) a smartwatch synced with a phone (b) the network joining a company's offices in Delhi and Chennai (c) the network inside one school building (d) the network joining the five branches of a college within one city.

Try it first, then show the solutionSolution

1. (a) Devices of one person within a few metres: PAN.
2. (b) Different cities: WAN.
3. (c) One building: LAN.
4. (d) Within one city: MAN.

Answer: (a) PAN (b) WAN (c) LAN (d) MAN

Topologies: bus, star and tree

NCERT section NCERT 10.5.3-10.5.5

A topology is the layout of the connections between nodes.

• Bus: all nodes attach to one main cable (the backbone) with terminators at both ends; data travels along the cable to all nodes. Advantages: least cable, easy to add a node. Disadvantages: a break in the backbone stops the whole network; slows down with heavy traffic; a fault is hard to locate.
• Star: every node has its own cable to a central hub or switch. Advantages: a broken cable affects only one node; easy to find faults and to add nodes. Disadvantages: more cable; if the central device fails, the whole network fails.
• Tree (hierarchical): several star networks joined to a common backbone, in levels like a tree. Advantages: easy to expand by adding branches; suits large organisations with departments. Disadvantages: a backbone failure cuts off branches; more cable and more complex to set up.

Worked example

A school wants each lab's computers on their own switch and the labs linked to the server room by one main cable. Which topology is this, and what happens if one lab's switch fails?

Try it first, then show the solutionSolution

1. Each lab is a star around its switch; the stars are joined to a common backbone: a tree topology.
2. If one lab's switch fails, only that lab's computers lose connection; the other branches keep working.

Answer: Tree topology; only the computers of that lab are cut off.

Web services: WWW, HTML, XML, URL and domain names

NCERT section NCERT 10.7.1, 10.8

The Internet is the network of networks (the hardware and connections); the World Wide Web (WWW) is the system of interlinked web pages that is accessed over the Internet using HTTP, invented by Tim Berners-Lee in 1990. HTML (HyperText Markup Language) uses predefined tags such as <h1> and <p> to describe how a page is displayed. XML (Extensible Markup Language) uses tags you define yourself (such as <student>) to store and carry data; it is about describing data, not displaying it, and its tags are case-sensitive and must be closed. A domain name is the readable name of a server, such as cbse.gov.in, which the DNS translates into its IP address. A URL (Uniform Resource Locator) is the full address of a resource: protocol + domain name + path, e.g. https://www.cbse.gov.in/results/index.html.

Worked example

In the URL https://www.ncert.nic.in/textbook/pdf/lecs110.pdf, identify the protocol, the domain name and the path. Which part is translated by DNS?

Try it first, then show the solutionSolution

1. Protocol: https.
2. Domain name: www.ncert.nic.in (top-level domain .in).
3. Path: /textbook/pdf/lecs110.pdf, the location of the file on that server.
4. DNS translates the domain name into the server's IP address.

Answer: https; www.ncert.nic.in; /textbook/pdf/lecs110.pdf; the domain name.

Website, web browser, web server and web hosting

NCERT section Not in the NCERT chapter; CBSE syllabus (web services)

A web page is one document on the web; a website is a collection of related web pages under one domain, with a home page. A web browser (Chrome, Firefox, Edge, Safari) is client software that requests pages using HTTP/HTTPS and displays them. A web server is a computer (and the software on it) that stores websites and sends pages to browsers when they ask. Web hosting is the service of keeping a website's files on a web server that is always connected to the Internet, so the site is available to everyone; a hosting company rents this space, and the domain name is pointed to the server.

Worked example

A student builds a website for the school fest on her laptop. Why can others not open it yet, and which two things does she need?

Try it first, then show the solutionSolution

1. The pages exist only on her laptop, which is not a web server reachable on the Internet all the time.
2. She needs web hosting (space on a web server that is always online) and a registered domain name pointing to that server.

Answer: It is not hosted; she needs web hosting and a domain name.

Computer Networks in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Network DevicesComing soon
• Network Topologies and Types of NetworksComing soon
• Web Services, URLs and Domain NamesComing soon
• Network Devices: Modem, Hub, Switch, Router, Gateway
• Network Types and Topologies: PAN, LAN, MAN, WAN, Bus, Star, Tree
• Web Services: WWW, HTML, XML, URL, Domain Name and Hosting

Computer Networks: syntax and key facts

Network types by area

PAN	about 10 m
LAN	up to about 1 km
MAN	about 30-40 km (a city)
WAN	country or world

Smallest to largest. The Internet is the largest WAN.

Device one-liners

Modem	digital ↔ analog
NIC	computer ↔ network
RJ45	cable connector
Repeater	boosts signal
Hub	sends to all
Switch	sends to one
Router	between networks
Gateway	between different protocols

Repeater rule of thumb: Ethernet cable runs beyond about 100 m need one.

Topologies

Bus: one backbone cable. Star: all to a central hub or switch. Tree: stars joined by a backbone in levels.

Only bus, star and tree are in the 2026-27 list.

URL parts

protocol :// domain name / path, e.g. https://www.cbse.gov.in/results/index.html

Domain name ≠ URL: the URL also has the protocol and the path.

HTML vs XML

HTML: predefined tags, displays data. XML: user-defined tags, stores and carries data.

XML tags are case-sensitive and must be closed.

How the board asks Computer Networks

The question patterns to expect, and the method that works for each.

Tricks to remember Computer Networks

Short memory aids. Say each one aloud once, then use it on a question the same day.

• "My New Robot Reads His Several Robots' Guides"

  Modem, NIC, RJ45, Repeater, Hub, Switch, Router, Gateway (plus the Wi-Fi card).

  Use it when: Listing the devices in the syllabus.

• Hub = loudspeaker, Switch = postman

  A loudspeaker announces to the whole room; a postman delivers to one house.

  Use it when: Hub vs switch questions.

• PLMW grows like your world

  Personal (you) → Local (school) → Metropolitan (city) → Wide (country and world).

  Use it when: Network types by area.

• Bus, Star, Tree = Road, Wheel, Family tree

  Bus: houses on one road; Star: spokes to a hub; Tree: branches of a family tree.

  Use it when: Drawing and describing the three topologies.

• HTML shows, XML stores

  HTML is how data looks; XML is what data is.

  Use it when: HTML vs XML.

Computer Networks: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 1 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Computer Networks

The slips examiners see most often, and what to write instead.

• ✗Mistake: Placing the server in the block nearest the centre instead of the block with the most computers

  The marking scheme follows the 80:20 traffic rule: most traffic is inside the biggest block, so the server should be there.

  ✓Do this: Write: 'Block X, because it has the maximum number of computers, so most traffic stays local.'

  Usually costs: 1 mark

• ✗Mistake: Saying a router connects computers within a lab

  Within a LAN the device is a hub or switch; a router joins different networks.

  ✓Do this: Lab or block: switch. Between networks or to the ISP: router. Different protocols: gateway.

  Usually costs: 1 mark

• ✗Mistake: Using the Internet and the WWW as the same thing

  The Internet is the global network itself; the WWW is one service on it (interlinked web pages over HTTP), alongside e-mail and file transfer.

  ✓Do this: In a difference question write one line for each and give e-mail as an Internet service that is not the web.

  Usually costs: 1 mark

• ✗Mistake: Answering mesh or ring for a topology question

  The 2026-27 syllabus lists bus, star and tree; the board's case answers use these.

  ✓Do this: Suggest star (or tree for many blocks) in design questions and justify it.

  Usually costs: 1 mark

• ✗Mistake: Writing the domain name when the URL is asked (or the reverse)

  A URL includes the protocol and the path; the domain name is only the server's name.

  ✓Do this: URL: https://www.cbse.gov.in/index.html. Domain name: www.cbse.gov.in (or cbse.gov.in).

  Usually costs: Half a mark

NCERT exercise guide: Computer Networks

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Computer Networks

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · Very short answerBoard level

   Expand: (a) NIC (b) XML (c) MAN (d) URL

   Hint

   Full forms.

   Answer

   (a) Network Interface Card (b) Extensible Markup Language (c) Metropolitan Area Network (d) Uniform Resource Locator

2. Q2 · 1 mark · MCQBoard level

   Which of these is not a network device? (a) Router (b) Switch (c) Compiler (d) Gateway

   Hint

   One of these translates programs.

   Answer

   (c) Compiler

3. Q3 · 2 marks · Short answerBoard level

   Give one advantage and one disadvantage of tree topology.

   Hint

   Expandability vs backbone dependence.

   Answer

   Advantage: easy to expand by adding branches; suits departments. Disadvantage: if the backbone fails, the branches are cut off; more cable and complex wiring.

4. Q4 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): A website can be accessed by anyone on the Internet only after it is hosted on a web server.
   Reason (R): A web browser stores websites and sends them to other computers.

   Hint

   Who stores and serves pages?

   Answer

   (c) A is true but R is false: a web server stores and serves pages; a browser requests and displays them.

5. Q5 · 2 marks · Short answerHarder board level

   Your home Wi-Fi router connects to the ISP through a telephone line. Which device on that line converts the signals, and why is it needed?

   Hint

   Digital vs analog.

   Answer

   A modem: computers use digital signals but the telephone line carries analog signals, so the modem modulates outgoing data and demodulates incoming data.

6. Q6 · 2 marks · Short answerHarder board level

   Differentiate between a website and a web page, and between a web server and a web browser.

   Hint

   Collection vs one document; serves vs requests.

   Answer

   A web page is a single document; a website is a set of related pages under one domain. A web server stores pages and sends them on request; a web browser requests pages and displays them.

7. Q7 · 2 marks · Short answerHOTS / competency

   A city's traffic police wants all 40 junction cameras across the city to send video to a control room. (a) Which type of network is needed? (b) Which topology would you suggest for cameras grouped by zone, with each zone's cameras on a zone switch? Justify.

   Hint

   City-wide; groups under switches.

   Answer

   (a) MAN (city-wide). (b) Tree: each zone is a star around its switch, and the zone switches connect to the control room's backbone, so new zones can be added easily.

8. Q8 · 2 marks · Short answerBoard level

   What is the role of DNS? Why are domain names used instead of IP addresses?

   Hint

   Phone book.

   Answer

   DNS translates a domain name into the IP address of the server. Domain names are easy to remember, while numeric IP addresses are not; the same name can also point to a new IP address if the server changes.

9. Q9 · 1 mark · Very short answerBoard level

   Name the network type and the likely connection for: (a) a laptop and a wireless mouse (b) all computers of a school (c) ATMs of a bank across India.

   Hint

   Area decides the type.

   Answer

   (a) PAN (Bluetooth or a small receiver) (b) LAN (Ethernet cables and switches, Wi-Fi) (c) WAN (leased lines or the Internet).

10. Q10 · 2 marks · Short answerHOTS / competency

    In the company with wings A (25), B (90), C (40), D (10) from the worked example, the management asks for the most economical cable layout (minimum total length) using these distances: A-B 80 m, B-C 60 m, B-D 110 m, A-C 120 m, A-D 70 m, C-D 140 m. Find it and give its total length.

    Hint

    Pick the shortest links that still connect all four blocks without a loop.

    Answer

    Take the shortest links in order: B-C 60 m, A-D 70 m, A-B 80 m. These three connect all four blocks: total 210 m (no link exceeds 100 m, so no repeater is needed).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Expand ARPANET and NSFNET.Show answer

   Advanced Research Projects Agency Network; National Science Foundation Network.

2. 2.Which device converts digital signals to analog and back?Show answer

   Modem.

3. 3.What is RJ45?Show answer

   An 8-pin connector used at the ends of Ethernet (twisted pair) cables.

4. 4.Which device connects networks that use different protocols?Show answer

   Gateway.

5. 5.Which topology fails completely if its main cable breaks?Show answer

   Bus topology.

6. 6.What is the approximate range of a PAN?Show answer

   About 10 metres.

7. 7.Give one difference between HTML and XML.Show answer

   HTML has predefined tags for displaying data; XML has user-defined tags for storing and carrying data.

8. 8.What does DNS do?Show answer

   Translates a domain name into the IP address of the server.

9. 9.What is web hosting?Show answer

   Storing a website's files on a web server that is always connected to the Internet so the site can be accessed by anyone.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Network: interconnected nodes sharing data and resources.
• ☐ARPANET (1969) → NSFNET (1986) → Internet (network of networks, TCP/IP).
• ☐Modem converts digital ↔ analog; NIC connects a computer to a network; RJ45 is the cable connector.
• ☐Repeater regenerates weak signals (Ethernet beyond about 100 m).
• ☐Hub broadcasts; switch forwards to one port; router joins networks; gateway joins different protocols.
• ☐PAN ~10 m; LAN up to ~1 km; MAN a city; WAN country or world.
• ☐Bus: one backbone; Star: central switch; Tree: stars on a backbone.
• ☐WWW = interlinked web pages over HTTP, on the Internet.
• ☐HTML displays (fixed tags); XML stores data (own tags).
• ☐URL = protocol + domain name + path; DNS maps domain names to IP addresses.
• ☐Web browser requests and shows pages; web server stores and sends them; hosting keeps a site online.

Computer Networks: frequently asked questions