The internet connects billions of devices across the globe, enabling real-time communication, streaming, banking, and collaboration at a scale unimaginable a few decades ago. But this remarkable infrastructure did not appear overnight — it evolved through decades of research, experimentation and incremental development. Understanding how computer networks evolved, what data communication actually means, how we measure network capacity, how devices are addressed on a network, and how data travels from source to destination forms the conceptual foundation for everything else in networking. For CBSE Class 12 Computer Science, Computer Networks carries 10 out of 70 theory marks — it is purely theoretical, highly predictable, and very scoring when prepared systematically.
1. Introduction to Computer Networks
A computer network is a collection of two or more computers and other hardware devices (printers, routers, switches) interconnected by communication links to share resources, data and information.
Benefits of Networking
| Benefit |
Explanation |
Example |
| Resource Sharing |
Hardware and software resources shared across users |
One printer shared by 20 computers in an office |
| Data/File Sharing |
Files and databases accessible to authorised users |
Students accessing shared study material on a school server |
| Communication |
Email, messaging, video calls across distances |
Video conferencing between two offices in different cities |
| Reduced Cost |
Shared resources reduce per-user hardware cost |
One high-speed internet connection shared by all employees |
| Reliability |
Data backed up across multiple systems |
If one server fails, another takes over |
2. Evolution of Networking — ARPANET, NSFNET, Internet
| Network |
Full Form |
Year |
Key Facts |
| ARPANET |
Advanced Research Projects Agency Network |
1969 |
Funded by the US Department of Defence (ARPA). First network to use packet switching. Connected 4 universities initially (UCLA, Stanford, UC Santa Barbara, University of Utah). Precursor to the modern Internet. |
| NSFNET |
National Science Foundation Network |
1986 |
Created by the US National Science Foundation to connect supercomputing centres. Extended networking to universities and research institutions across the US. Replaced ARPANET as the backbone of the Internet in the US. Decommissioned in 1995 when commercial networks took over. |
| Internet |
Interconnected Network |
1991 (WWW) |
A global network of networks using TCP/IP protocol suite. The World Wide Web (WWW) was introduced in 1991 by Tim Berners-Lee making the Internet user-friendly. Internet came to India in 1995 (launched by VSNL). Today connects billions of devices worldwide. |
Key Milestones in Networking Evolution
- 1969: ARPANET — first 4-node network; introduced packet switching concept.
- 1971: First email sent over ARPANET (by Ray Tomlinson — first use of @ in email addresses).
- 1973: TCP/IP protocols developed by Vint Cerf and Bob Kahn.
- 1983: ARPANET switched to TCP/IP — considered the "birth of the Internet."
- 1986: NSFNET created to extend networking to academics.
- 1991: Tim Berners-Lee introduced the World Wide Web — HTML, HTTP, URL.
- 1993: Mosaic — first graphical web browser — made WWW accessible to general public.
- 1995: Internet introduced in India by VSNL; NSFNET decommissioned.
3. Data Communication — Concepts and Components
Data communication is the process of transferring data between two or more devices over a transmission medium (guided or unguided).
Components of Data Communication
| Component |
Role |
Example |
| Sender |
Device that originates and sends the message |
Your computer when sending an email |
| Receiver |
Device that receives the transmitted message |
Recipient's computer/phone receiving the email |
| Message |
The actual data/information being transmitted |
Text, images, audio, video, files |
| Communication Media |
The physical path through which the message travels |
Ethernet cable, fibre-optic cable, Wi-Fi signal |
| Protocols |
Set of rules governing how data is transmitted and received |
TCP/IP, HTTP, FTP — defines format, timing, error control |
Common Networking Terms
| Term |
Definition |
| Node |
Any device connected to a network (computer, printer, router, smartphone) |
| Workstation |
A computer connected to a network used by an individual user |
| Server |
A powerful computer that provides services (files, databases, web pages) to other nodes on the network |
| Client |
A device that requests and uses services provided by a server |
| Protocol |
A set of rules/standards that govern how data is transmitted between devices on a network |
| Domain Name |
Human-readable name mapped to an IP address (e.g., www.google.com) |
4. Measuring Network Capacity — Bandwidth and Data Transfer Rate
| Term |
Definition |
Unit |
| Bandwidth |
The maximum capacity of a communication channel to transmit data — the theoretical upper limit of data transfer |
bps, Kbps, Mbps, Gbps |
| Data Transfer Rate (DTR) |
The actual speed at which data is successfully transferred from sender to receiver in practice — always ≤ Bandwidth |
bps, Kbps, Mbps, Gbps |
Units of Data Transfer
| Unit |
Full Form |
Value |
| bps |
Bits per second |
Base unit |
| Kbps |
Kilobits per second |
1,000 bps |
| Mbps |
Megabits per second |
1,000 Kbps = 10⁶ bps |
| Gbps |
Gigabits per second |
1,000 Mbps = 10⁹ bps |
Analogy: Bandwidth is like the width of a highway (maximum lanes of traffic); Data Transfer Rate is the actual traffic flow at any given time.
5. IP Address
An IP (Internet Protocol) Address is a unique numerical label assigned to every device connected to a network that uses the Internet Protocol for communication. It serves two main purposes: host identification and location addressing.
| Version |
Bits |
Format |
Example |
Total Addresses |
| IPv4 |
32-bit |
4 octets separated by dots (decimal) |
192.168.1.1 |
≈ 4.3 billion (2³²) |
| IPv6 |
128-bit |
8 groups of 4 hex digits separated by colons |
2001:0db8:85a3::8a2e:0370:7334 |
≈ 3.4 × 10³⁸ (2¹²⁸) |
- IPv4 addresses are running out — IPv6 was developed to solve this.
- Each octet in IPv4 ranges from 0 to 255 (8 bits).
- IP addresses are logical addresses (software-assigned, can change).
- MAC (Media Access Control) Address is the physical address — permanently burned into the network card (48-bit hexadecimal).
6. Switching Techniques
Switching is the method by which data is routed from the sender to the receiver through a network. There are two main techniques in the CBSE syllabus:
| Feature |
Circuit Switching |
Packet Switching |
| Definition |
A dedicated physical path is established between sender and receiver before data transmission begins |
Data is broken into small packets; each packet may travel a different route independently and is reassembled at the destination |
| Path |
Fixed, dedicated path throughout communication |
No fixed path — each packet routed independently |
| Resource usage |
Resources reserved even if not in use (wasteful) |
Resources used only when needed (efficient) |
| Setup time |
Time required to establish the circuit before data flows |
No setup time — packets sent immediately |
| Delay |
Once circuit established, delay is minimal and consistent |
Variable delay — packets may arrive out of order |
| Failure handling |
If the path fails, the entire connection breaks |
Packets can be rerouted around failures |
| Best for |
Voice calls — requires consistent, uninterrupted connection |
Internet data — emails, web pages, files |
| Example |
Traditional telephone (PSTN) calls |
Internet — the entire web uses packet switching |
What is a Packet?
In packet switching, data is divided into small chunks called packets. Each packet contains:
- Header: Source IP, Destination IP, packet number, total packets — used for routing and reassembly.
- Payload (Data): The actual fragment of the original message.
- Trailer: Error detection information (checksum).
Packets are independently routed, may arrive out of order, and are reassembled at the destination. ARPANET was the first network to use packet switching.
Practice Questions (CBSE Board Level)
Q1 (1 mark): Expand the abbreviation ARPANET. Why is it significant in the history of the Internet?
ARPANET: Advanced Research Projects Agency Network.
Significance: ARPANET (launched in 1969) was the first wide-area network funded by the US Department of Defense. It is considered the precursor to the modern Internet because it introduced the concept of packet switching—the core technology that underlies the entire Internet today. It was the first network to connect multiple remote universities for resource sharing and data communication.
Q2 (2 marks): Differentiate between Bandwidth and Data Transfer Rate.
Bandwidth is the maximum capacity of a communication channel to transmit data—the theoretical upper limit of how much data can be sent per second. It is measured in bps, Kbps, Mbps, or Gbps.
Data Transfer Rate (DTR) (often called throughput) is the actual speed at which data is successfully transferred from sender to receiver in practice. Due to network congestion, interference, and protocol overhead, DTR is always less than or equal to the bandwidth.
Analogy: Bandwidth is the maximum number of lanes on a highway; DTR is the actual traffic flow at a given time.
Q3 (2 marks): Differentiate between Circuit Switching and Packet Switching.
| Basis |
Circuit Switching |
Packet Switching |
| Path |
A dedicated, fixed path is established before communication begins. |
No fixed path exists; each data packet is routed independently. |
| Resource Use |
Resources are reserved for the duration of the connection, even when idle (wasteful). |
Resources are shared and only used dynamically when transmitting (highly efficient). |
| Best For |
Continuous voice calls (traditional telephone networks/PSTN). |
Internet data (email, web browsing, file transfers). |
Q4 (1 mark): What is an IP address? Write the difference between IPv4 and IPv6 in terms of the number of bits used.
An IP (Internet Protocol) address is a unique numerical label assigned to every device connected to a computer network that uses the Internet Protocol for communication. It is used for host identification and location addressing.
IPv4 uses 32 bits—written as four decimal octets (e.g., 192.168.1.1), providing approximately 4.3 billion addresses.
IPv6 uses 128 bits—written in hexadecimal format (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334), providing approximately 3.4 × 10³⁸ addresses. IPv6 was developed strictly because the world ran out of available IPv4 addresses.
Q5 (2 marks): What is the World Wide Web (WWW)? Who invented it and when? Is WWW the same as the Internet? Justify.
The World Wide Web (WWW) is an information system consisting of interlinked hypertext documents and multimedia content that are accessed via web browsers over the Internet. It was invented by Tim Berners-Lee in 1989/1991.
WWW is NOT the same as the Internet. The Internet is the massive, global physical network infrastructure (hardware: cables, routers, servers) that connects billions of computers worldwide. The WWW is simply an application or service that runs on top of the Internet, using HTTP to deliver web pages. Other Internet services, like email (SMTP) and file transfer (FTP), exist entirely independently of the WWW.
Q6 (3 marks): List any three components of data communication and explain each briefly with an example.
Explanation (Any three):
- Sender: The device that originates and transmits the message. Example: Your laptop when you press 'send' on an email—it initiates the data transmission.
- Receiver: The device that is the intended target of the message. Example: The mail server or your friend's smartphone that ultimately receives the email.
- Communication Medium (Channel): The physical or wireless path over which the data travels from sender to receiver. Example: A fiber-optic cable, copper wire, or a Wi-Fi signal.
- Protocol: A strict set of rules governing how data is formatted, transmitted, received, and acknowledged across a network. Example: TCP/IP is the protocol suite for the Internet—it defines exactly how data is broken into packets, addressed, and reassembled at the destination.
Q7 (1 mark): In which year was the Internet introduced in India and by which organisation?
The Internet was officially introduced for public access in India on August 15, 1995, by VSNL (Videsh Sanchar Nigam Limited).
