Computer Networks Class 12 Notes — Chapter 10 Revision Guide

If you are revising the night before a test, start here: a computer network is simply an interconnection of two or more computers or computing devices that lets them share data and resources (NCERT, p. 182).

These Computer Networks Class 12 notes compress Chapter 10 of the NCERT Computer Science textbook into the essentials — network types from PAN to WAN, networking devices, topologies, MAC and IP addressing, the Internet versus the Web, and DNS — with the tables, formulas and worked examples you need for a quick pass.

The notes follow the 2026-27 session and stick to the rationalised NCERT text, which you can always verify on the official NCERT textbook portal. They are part of the Class 12 Computer Science notes collection, built for revision rather than first-time reading.

Computer Networks Chapter at a Glance

This chapter builds one idea on another in a fixed order. Keep this chain in mind while revising:

• Computer network — two or more computers or computing devices interconnected to share data and resources (p. 182).
• Evolution — began with the ARPANET research project of the 1960s and grew into the Internet (p. 183).
• Types — classified by geographical area covered and data transfer rate: PAN, LAN, MAN, WAN (p. 184).
• Devices — modem, Ethernet card/NIC, RJ45, repeater, hub, switch, router and gateway connect and manage nodes (p. 187–191).
• Topologies — the arrangement of computers and peripherals: mesh, ring, bus, star and tree (p. 191).
• Addressing — every node is identified by a permanent MAC address and a changeable IP address (p. 194).
• Internet, Web, DNS — the Internet is the global network, the Web is the interlinked pages on it, and DNS converts domain names to IP addresses (p. 195–198).

If you are revising the complete book, keep these notes with the related database concepts and SQL notes from this subject.

Two figures give the chapter’s core picture. Figure 10.1 shows a social network — people as interconnected nodes — which is exactly what the word “network” means: a set of two or more similar things or people interconnected with each other (p. 182).

When the members are computers instead of people, you get the computer network of Figure 10.2: computers and hosts such as printers connected so they can share data and resources (p. 182).

Key Terms You Must Know: Definitions with Examples

These are the chapter’s load-bearing terms. Learn the meaning and one example for each, and most questions in the chapter become manageable.

From ARPANET to the Internet: Evolution of Networking

In the 1960s, the Advanced Research Projects Agency Network (ARPANET) — a research project commissioned in the U.S. Department of Defence — set out to connect academic and research institutions located at different places for scientific collaboration. The first message was communicated between the University of California, Los Angeles (UCLA) and the Stanford Research Institute (SRI).

More organisations joined ARPANET over time, many independent smaller networks formed, and these networks gradually interconnected into today’s Internet (NCERT, p. 183).

Figure 10.3 is a timeline of the milestones in this journey. For revision, remember the two facts above — the project’s origin and the first message — and the idea that independent networks merged rather than one network growing alone.

PAN, LAN, MAN and WAN: Network Types Compared

Computer networks are classified by geographical area covered and data transfer rate (NCERT, p. 184). Learn the four types in order of growing size: PAN, LAN, MAN, WAN.

Figure 10.4 shows a PAN: a few personal devices around one user. Figure 10.5 shows a LAN: devices inside one building or campus. Figure 10.6 shows a MAN spreading across a city and linking several local networks. Figure 10.7 shows a WAN joining LANs across distant cities and countries.

Decoding the textbook’s Think and Reflect: you can reach your bank account from anywhere in the world — is the bank’s network a LAN, MAN or WAN? Work it out layer by layer.

Each branch runs a LAN for its local devices; the branches are interconnected across cities, so the bank’s own backbone is a WAN; and customers reach their accounts through the Internet, which is itself the largest WAN (p. 186).

Network Devices: What Each One Does (Modem to Gateway)

These devices sit between computers and the transmission medium, and each has one specific job. Learn the table first, then look at the figures.

An ISP (Internet Service Provider) is any organisation that provides services for accessing the Internet (p. 189). The signals these devices handle travel through transmission media, which the Class 12 data communication notes cover.

Hub versus Switch: the comparison that matters

The hub and the switch both connect multiple computers, but they behave completely differently when data flows (NCERT, p. 189).

The figures below show what these devices look like in practice.

Figure 10.8 shows the modem’s role: the computer’s digital data is converted into an analog signal for the medium, and a modem at the other end converts it back.

Figure 10.9 shows the NIC — the circuit board mounted on the motherboard that acts as the interface between the computer and the wired network. Figure 10.10 shows the RJ45, the eight-pin plastic plug at the end of every Ethernet cable that fits into the NIC’s jack.

Figure 10.12 shows Ethernet cables connected to a switch — the device a home or office uses to create a LAN or reach the Internet.

Figure 10.13 shows a router — frequently wireless — that gives Wi-Fi access to devices and usually combines the jobs of a router, modem and switch.

Figure 10.14 shows a gateway sitting at the edge of the organisation’s network — every packet entering or leaving passes through it.

Network Topologies: Mesh, Ring, Bus, Star and Tree

A topology is the arrangement of computers and other peripherals in a network (p. 191). The chapter covers five: mesh, ring, bus, star and tree.

Bus and Ring when a node goes down (Think and Reflect, p. 193)

• Bus: a node that fails does not break the shared backbone — the remaining nodes keep communicating. The network dies only if the backbone cable itself breaks.
• Ring: data flows unidirectionally and must pass through every node, so a failed node breaks the ring and transmission stops.

Figure 10.15 shows a mesh: every node with its own direct cable to every other node — that is why the wiring is complex and expensive.

Figure 10.16 shows a ring: each node connected to exactly two neighbours, forming a closed loop.

Figure 10.17 shows a bus: all nodes tapping into one backbone cable.

Figure 10.19 shows a hybrid or tree topology: four star topologies joined by a bus — a realistic combination used when multiple LANs are connected.

The textbook’s star topology figure (10.18) is represented here in prose: every device connects by its own cable to a central hub or switch, and only the central device’s failure brings the network down (p. 193). If a question asks you to draw or identify a star, look for the central node and the independent cables.

The Key Formula: Cables Needed in a Mesh Topology

A fully connected mesh of \( n \) nodes needs exactly \[ \text{Number of cables} = \frac{n(n-1)}{2} \]

where \( n \) = number of nodes (devices) in the mesh, and the unit of the answer is cables or wires (NCERT, p. 192).

Why \( \frac{n(n-1)}{2} \)? Each node must have a cable to every other node, so \( n \) nodes each need \( n-1 \) cables — that gives \( n(n-1) \) links. But every link is counted twice (once from each end), so divide by 2.

Compare with star: this formula applies only to mesh. In a star, each device needs exactly one cable to the central device, so \( n \) nodes need only \( n \) cables.

Worked Examples: Cable Count in Mesh vs Star

Example A: Mesh with 6 nodes

Example B: 10 nodes — mesh versus star

The mesh needs 4.5 times more cabling than a star for the same 10 nodes. That gap is exactly why mesh is so reliable — many alternative routes exist — but also why it is the most expensive topology (p. 192).

MAC Address vs IP Address: How Networks Identify Nodes

A network must tell the sender from the receiver before it can route data. It does this with two addresses: the permanent MAC and the changeable IP (p. 194).

Why both? The MAC says which machine; the IP says where it is in the internetwork. Because the IP can change every time the node moves networks, the MAC keeps a stable identity, and routing uses the IP to reach the right network (p. 194–195).

IPv4 offered just under 4.3 billion unique addresses, which proved too few as devices multiplied — that is why the 128-bit IPv6 was proposed.

Internet vs World Wide Web: Spot the Difference

This is the chapter’s most common conceptual confusion: the Internet is the infrastructure, and the Web is a service running on it (p. 196–197).

Analogy: the Internet is the road network; the Web is the shops you visit on those roads. Roads can exist without shops, but no shop is reachable without a road.

How data reaches the Internet: computers connect to a modem by cable or Wi-Fi, the modem connects to a local ISP, local ISPs join into regional networks, regional networks form national networks, and country-wise networks form the Internet backbone (p. 195).

How the Web was born: Sir Tim Berners-Lee, a British computer scientist, invented the World Wide Web in 1990 by defining three fundamental technologies (NCERT, p. 196):

• HTML (HyperText Markup Language) — designs standardised web pages so content can be read from any computer.
• URL/URI (Uniform Resource Locator / Identifier) — the unique address of each resource on the web.
• HTTP (HyperText Transfer Protocol) — the set of rules used to retrieve linked web pages; HTTPS is the more secure version.

Internet of Things (IoT) extends the same idea: smart appliances such as TVs, air conditioners, refrigerators and security cameras also communicate through networks — a topic introduced in Class 11 (p. 195).

Domain Name System: How Names Become IP Addresses

Every website is stored on a server connected to the Internet, and every server has an IP address — but nobody wants to remember strings of numbers. So each server is given a name — a domain name or hostname — mapped to its unique IP address (p. 197).

Think of it as a phonebook: instead of remembering phone numbers, we assign names to them (p. 197). Real mappings from the textbook include ncert.nic.in → 164.100.60.233 and wikipedia.org → 198.35.26.96 (p. 197). You can spot the pattern: the IP number is hard to remember, the name is not.

Domain name resolution is the conversion of a domain name to its corresponding IP address, and it is done by a DNS server (NCERT, p. 198).

The step-by-step: what happens when you enter a URL

1. You enter a URL in the web browser.
2. The browser’s HTTP protocol asks a DNS server for the IP address of that domain name.
3. The DNS server returns the IP address.
4. HTTP retrieves the information using that IP and loads it in your browser.

Figure 10.20 shows exactly this exchange: HTTP requests the IP corresponding to a domain name, and the DNS server sends the IP back.

How DNS servers are organised: DNS servers are placed in hierarchical order. At the top are 13 root servers named using the letters A through M; the Internet Assigned Numbers Authority (IANA) keeps the list of root servers.

Below the roots sit other DNS servers at different levels, each either holding the IP for a domain or pointing to another server where the entry can be found (p. 198).

Common Mistakes in Computer Networks (and the Corrections)

These are the chapter-specific slips that cost students. Use the third column as a built-in check.

Exam Notes: How to Write Complete Answers to This Chapter

The chapter ends with sixteen exercises (p. 201–202). They fall into a few answer types; learn the shape of each and you can answer any variant.

Revision Summary: Computer Networks in One Page

Definitions in one line: a computer network connects two or more computers or computing devices to share data and resources; every participating device is a node; data travels as packets (p. 182).

Memory trick: “room, building, city, world” maps exactly to PAN, LAN, MAN, WAN — small to large.

Device one-liners:

• Modem: converts digital ↔ analog.
• NIC (Ethernet card): wired adapter; holds the MAC address.
• RJ45: eight-pin plug on Ethernet cables.
• Repeater: regenerates weak signals (beyond about 100 m).
• Hub: broadcasts to all lines → collisions.
• Switch: forwards to the destination only; drops corrupted signals.
• Router: connects different networks; can repackage data.
• Gateway: entry and exit of a network; firewall integrated.

Topology one-liners:

• Mesh: all-to-all; reliable, secure, costly; \( \frac{n(n-1)}{2} \) cables.
• Ring: two neighbours; unidirectional flow.
• Bus: shared backbone; cheap and easy to maintain.
• Star: central hub or switch; central failure kills it.
• Tree/hybrid: branches of basic topologies; seen in WANs.

MAC vs IP: MAC is permanent, physical, 48-bit hexadecimal, engraved on the NIC; IP is changeable and logical — IPv4 is 32-bit dotted-decimal, IPv6 is 128-bit hexadecimal groups.

Internet vs Web: Internet = global network of devices; Web = interlinked pages on it. DNS is the phonebook that converts domain names to IP addresses.

Exam-ready sentence: in any scenario question, identify the network, device or topology first, then justify it with one grounded fact — the type’s coverage, the device’s key job, or the topology’s defining clue.

For the rest of your revision, browse all Class 12 notes or the full CBSE notes library.

FAQs: Computer Networks Class 12 Doubts

Is the Internet a WAN?

Yes. A WAN connects LANs and MANs spread across different geographical locations, and the Internet is the largest WAN because it connects billions of computers, smartphones and millions of LANs from different continents (p. 186).

What is the difference between a hub and a switch?

A hub broadcasts every arriving frame to all other lines, so simultaneous data from two devices collides. A switch reads the destination address from each packet, looks it up in a table, and forwards only to the node that needs it; it can forward multiple packets at once and drops corrupted signals (NCERT, p. 189).

Why does a device need both a MAC address and an IP address?

The MAC is permanent and identifies the device itself on the local network; the IP is logical and identifies where the node is in the internetwork. Because the IP can change when a node moves networks, the MAC keeps local identification stable while the IP delivers data across networks (p. 194–195).

How many cables are needed to connect 8 nodes in a mesh topology?

Use the mesh formula \( \frac{n(n-1)}{2} \) with \( n = 8 \): \( \frac{8 \times 7}{2} = 56 \div 2 = 28 \). So a fully connected mesh of 8 nodes needs 28 cables.

What is the difference between the Internet and the World Wide Web?

The Internet is the global network of interconnected computing devices; the Web is the interlinked collection of web pages and resources accessible over that network. Analogy: the Internet is the road network, and the Web is the shops you visit on those roads (p. 195–197).

What happens when you type a URL in the browser? How does DNS help?

Your browser’s HTTP protocol asks a DNS server for the IP address of that domain name; the DNS server returns the IP; HTTP then retrieves the web resource using that IP and loads it in your browser. This conversion of a domain name to an IP address is called domain name resolution (p. 198).

Reference: NCERT Class 12 Computer Science textbook, chapter Computer Networks.