CBSE CLASS XII – COMPUTER SCIENCE (083)
UNIT 2: COMPUTER NETWORKS
1. Introduction to Computer Networks
A computer network is a collection of two or more computers and other devices connected together to share:
Data
Information
Hardware resources
Software resources
Internet connection
Printers and other devices
Simple Definition
A computer network is a group of interconnected computing devices that can communicate and share resources with each other.
Simple Network Structure
Computer 1
|
|
+----+----+
| Network |
| Device |
+----+----+
|
+------+------+
| |
Computer 2 Computer 3
Example
In a school computer laboratory, 30 computers may be connected to a network.
Students can:
Share files.
Access the Internet.
Use a common printer.
Access a common server.
Share other network resources.
2. Why Do We Need Computer Networks?
Computer networks provide many benefits.
1. Resource Sharing
Multiple computers can use a common printer or storage device.
Computer 1 ─┐
Computer 2 ─┼── Network ── Printer
Computer 3 ─┘
2. File Sharing
Users can transfer files from one computer to another.
3. Communication
People can communicate using:
Email
Chat
Video conferencing
VoIP
4. Internet Sharing
Many computers can access the Internet through a common network connection.
5. Centralised Management
A server can manage users, files, applications, and security.
3. Evolution of Networking
Computer networking developed gradually.
A simplified history is:
ARPANET
↓
NSFNET
↓
Internet
↓
Modern Internet
↓
Web, Cloud, Mobile, IoT, 5G and beyond
The three important milestones in the CBSE syllabus are:
ARPANET
NSFNET
Internet
4. ARPANET
ARPANET stands for:
Advanced Research Projects Agency Network
It was one of the earliest large-scale computer networks and became an important foundation for the modern Internet.
It was developed in the United States under the Advanced Research Projects Agency (ARPA).
Important Point
ARPANET was designed to allow computers at different locations to communicate and share resources.
Basic Structure
University A
|
|
ARPANET
/ | \
/ | \
Lab A Lab B University B
Importance of ARPANET
Connected geographically separated computers.
Helped develop networking technologies.
Played a major role in the development of the Internet.
Demonstrated packet-based computer communication.
5. NSFNET
NSFNET stands for:
National Science Foundation Network
It was developed by the U.S. National Science Foundation (NSF).
NSFNET connected research and educational institutions and became an important part of the growth of large-scale networking.
Simplified Structure
NSFNET
/ | \
/ | \
University University Research
A B Centre
NSFNET helped expand networking beyond the earlier research networks and contributed to the growth of the Internet.
6. Internet
The Internet is a worldwide interconnected system of computer networks.
It connects:
Computers
Servers
Smartphones
Routers
IoT devices
Data centres
Other network-enabled devices
Internet Structure
Computer ─┐
Laptop ───┤
Mobile ───┤
Tablet ───┼── Router ── ISP ── Internet ── Websites
Server ───┤
│
Network
Important
The Internet is often described as a "network of networks" because many different networks are interconnected.
7. ARPANET vs NSFNET vs Internet
| Feature | ARPANET | NSFNET | Internet |
|---|---|---|---|
| Full form | Advanced Research Projects Agency Network | National Science Foundation Network | Common name for global interconnected networks |
| Main role | Early research networking | Expanded academic/research networking | Worldwide networking |
| Scope | Initially limited | Larger academic/research infrastructure | Global |
| Importance | Foundation for later networking | Helped expand large-scale networking | Modern global network |
Remember
ARPANET → Early networking foundation
NSFNET → Major expansion of academic/research networking
Internet → Global network of networks
8. Data Communication
Data communication is the process of transferring data from one device to another through a communication medium.
Example
When you send a message from your smartphone:
Your Mobile
|
| Data
v
Network
|
v
Internet
|
v
Receiver's Mobile
The message travels from the sender to the receiver through communication networks.
9. Components of Data Communication
The five basic components are:
Sender
Receiver
Message
Communication Media
Protocol
Figure
MESSAGE
|
v
SENDER ── Communication Media ──> RECEIVER
|
Protocols
10. Sender
The sender is the device that sends the data.
Examples:
Computer
Smartphone
Server
Tablet
Example
If Rahul sends an email to Amit:
Rahul's Computer → Sender
11. Receiver
The receiver is the device that receives the data.
Example:
Amit's Computer → Receiver
12. Message
The message is the actual information being transmitted.
It may be:
Text
Image
Audio
Video
File
Email
Example:
"Hello Amit"
is the message.
13. Communication Media
Communication media is the path through which data travels from sender to receiver.
It can be:
Wired
Twisted pair
Coaxial cable
Fibre-optic cable
Wireless
Radio waves
Microwaves
Infrared waves
14. Protocol
A protocol is a set of rules that controls communication between devices.
Just as people need a common language and rules for communication, computers need protocols.
Examples:
HTTP
FTP
SMTP
TCP/IP
HTTPS
POP3
TELNET
15. Data Communication Model
+---------+ +-------------------+ +----------+
| SENDER | ----> | Communication | ----> | RECEIVER |
| | | Medium + Protocol | | |
+---------+ +-------------------+ +----------+
|
MESSAGE
16. Bandwidth
Bandwidth refers to the capacity of a communication channel to carry data.
It is commonly measured in:
bits per second (bps)
Kbps
Mbps
Gbps
Common Units
1 Kbps = 1,000 bits per second
1 Mbps = 1,000,000 bits per second
1 Gbps = 1,000,000,000 bits per second
In networking, the decimal convention is commonly used for these rate units.
Example
A network connection with a data rate of:
100 Mbps
can theoretically transmit up to 100 million bits per second under the stated conditions.
17. Data Transfer Rate
Data transfer rate is the amount of data transferred from one point to another in a given amount of time.
It is usually measured in:
bits per second (bps)
Formula
Data Transfer Rate = Amount of Data / Time
Example
Suppose 50 MB of data is transferred in 5 seconds.
Conceptually:
Rate = Data / Time
Actual network performance may differ because of protocol overhead, congestion, latency and other factors.
18. Bandwidth vs Data Transfer Rate
| Bandwidth | Data Transfer Rate |
|---|---|
| Capacity of a communication channel | Actual/observed rate of data transfer |
| Indicates how much data a channel can carry | Indicates how fast data is transferred |
| Often expressed in bps | Usually expressed in bps |
| Higher bandwidth can support higher transfer rates | Can be affected by many network conditions |
19. IP Address
IP stands for:
Internet Protocol
An IP address is a numerical address used to identify a device/interface on an IP network.
It helps data reach the correct destination.
20. IPv4 Address
IPv4 uses a 32-bit address.
It is normally written as four decimal numbers separated by dots.
Example:
192.168.1.10
Each part is called an octet.
192 . 168 . 1 . 10
| | | |
Octet Octet Octet Octet
Each IPv4 octet ranges from:
0 to 255
21. IPv6 Address
IPv6 uses 128-bit addresses.
Example:
2001:db8:1234::1
IPv6 provides a much larger address space than IPv4.
IPv4 vs IPv6
| IPv4 | IPv6 |
|---|---|
| 32-bit | 128-bit |
Example 192.168.1.10 | Example 2001:db8::1 |
| Smaller address space | Much larger address space |
| Dotted decimal notation | Hexadecimal colon notation |
22. Switching Techniques
Switching is the process of transferring data from the source to the destination through a network.
Two important switching techniques are:
Circuit Switching
Packet Switching
23. Circuit Switching
In circuit switching, a dedicated communication path is established between sender and receiver before communication begins.
Example
Traditional telephone networks are a classic example.
Structure
Sender
|
A
|
B
|
C
|
Receiver
Dedicated Path
The path remains reserved during the communication session.
Characteristics
Dedicated path
Connection established before communication
Resources remain reserved during the session
Predictable path
Can be inefficient when the connection is idle
24. Packet Switching
In packet switching, data is divided into small units called packets.
Packets may travel through different routes and are reassembled at the destination.
Structure
+---- Router A ----+
/ \
Sender ----< >---- Receiver
\ /
+---- Router B ----+
Different packets can potentially follow different paths.
Example
The Internet primarily uses packet-based communication.
25. Circuit Switching vs Packet Switching
| Circuit Switching | Packet Switching |
|---|---|
| Dedicated path | No permanently dedicated path |
| Path established before transmission | Data is divided into packets |
| Resources may remain reserved | Network resources are shared |
| Suitable for continuous dedicated communication | Efficient for bursty data communication |
| Traditional telephone networks are a classic example | Internet uses packet-based networking |
26. Transmission Media
Transmission media is the medium through which data travels.
It is divided into:
TRANSMISSION MEDIA
|
+---------+---------+
| |
WIRED WIRELESS
| |
+------+------+ +----+-----+
| | | | | |
Twisted Coaxial Fibre Radio Micro Infrared
Pair Cable Optic Waves waves Waves
27. Wired Communication Media
Wired media uses a physical cable.
The important types are:
Twisted Pair Cable
Coaxial Cable
Fibre-Optic Cable
28. Twisted Pair Cable
A twisted pair cable consists of pairs of insulated copper wires twisted together.
Figure
Wire A ~~~~~~~~~~~~~~~~~
\ / \ / \ / \ /
Wire B ~~~~~~~~~~~~~~~~~
The twisting helps reduce electromagnetic interference.
Types
UTP – Unshielded Twisted Pair
STP – Shielded Twisted Pair
Advantages
Low cost
Easy to install
Commonly used in Ethernet networks
Disadvantages
More susceptible to interference than fibre
Limited distance compared with fibre
Uses
LAN
Telephone systems
Ethernet connections
29. Coaxial Cable
A coaxial cable contains a central conductor surrounded by insulation and shielding.
Structure
+--------------------------------+
| Outer Protective Jacket |
| +--------------------------+ |
| | Metallic Shield | |
| | +--------------------+ | |
| | | Insulator | | |
| | | +-------------+ | | |
| | | | Conductor | | | |
| | | +-------------+ | | |
| | +--------------------+ | |
| +--------------------------+ |
+--------------------------------+
Advantages
Better shielding than basic twisted pair
Can carry signals over longer distances than some basic copper links
Durable
Uses
Cable television
Broadband systems
Some communication systems
30. Fibre-Optic Cable
Fibre-optic cable uses light to transmit data through optical fibres.
Structure
+--------------------------+
| Protective Jacket |
| +------------------+ |
| | Optical Fibre | |
| | Core | |
| +------------------+ |
+--------------------------+
Data Transmission
Electrical Data
|
v
Light Signals
|
v
Fibre Cable
|
v
Light Detector
|
v
Electrical Data
Advantages
Very high bandwidth
High-speed communication
Low signal loss over long distances
Resistant to electromagnetic interference
More difficult to tap physically than many copper links
Disadvantages
Higher installation cost
More delicate than ordinary copper cables
Installation and repair require specialised equipment
Uses
Internet backbone
Long-distance communication
Undersea cables
High-speed networks
31. Wired Media Comparison
| Feature | Twisted Pair | Coaxial | Fibre Optic |
|---|---|---|---|
| Main medium | Copper wires | Copper conductor | Glass/plastic optical fibre |
| Signal | Electrical | Electrical | Light |
| Cost | Low | Moderate | Higher |
| Bandwidth | Moderate | Higher than basic twisted pair in many applications | Very high |
| EMI resistance | Lower | Better shielding | Excellent |
| Common use | LAN/Ethernet | Cable TV/broadband | High-speed backbone |
32. Wireless Communication Media
Wireless media transmits data without a physical cable.
Important types:
Radio Waves
Microwaves
Infrared Waves
33. Radio Waves
Radio waves are electromagnetic waves used for wireless communication.
They can travel through air and can cover relatively large areas depending on frequency, power, antennas and environment.
Uses
Radio broadcasting
Wireless networking
Mobile communication
Bluetooth and other wireless systems
Advantages
No physical cable required
Can cover broad areas
Useful for mobile devices
Disadvantages
Can suffer from interference
Security requires proper protection
Performance depends on environment and frequency
34. Microwaves
Microwaves are high-frequency electromagnetic waves.
They are commonly used for:
Point-to-point communication
Satellite communication
Cellular networks
Long-distance wireless links
Important Feature
Many terrestrial microwave systems require a reasonably clear line of sight between antennas.
Figure
Antenna A )))))))))))))) Antenna B
Microwave
Link
35. Infrared Waves
Infrared communication uses infrared electromagnetic radiation.
It is generally used for short-range communication.
Examples
Remote controls
Short-range device communication
Some sensors
Characteristics
Short range
Usually requires a relatively clear path
Does not normally pass through walls effectively
36. Wireless Media Comparison
| Radio Waves | Microwaves | Infrared |
|---|---|---|
| Broad range of frequencies | Higher-frequency wireless communication | Infrared spectrum |
| Can cover large areas | Often directional | Usually short range |
| Used in radio/mobile/wireless systems | Satellite and point-to-point links | Remote controls |
| Can penetrate some obstacles depending on frequency | Often needs line of sight | Usually cannot pass through walls effectively |
37. Network Devices
Important network devices are:
Modem
Ethernet Card
RJ45
Repeater
Hub
Switch
Router
Gateway
Wi-Fi Card
38. Modem
Modem stands for:
Modulator-Demodulator
A modem converts signals between forms suitable for transmission over a particular access medium.
Historically, the term refers to modulation/demodulation of signals, although modern broadband devices may integrate several networking functions.
Basic Idea
Computer
|
Digital Data
|
Modem
|
Transmission Network
Uses
Internet connectivity over certain access technologies
Communication between a local network and service provider equipment
39. Ethernet Card
An Ethernet card is a network interface that allows a computer or other device to connect to an Ethernet network.
It may be:
Built into the motherboard
Added as an expansion card
Example
Computer
|
Ethernet Port
|
Network Cable
|
Switch
40. RJ45
RJ45 is commonly used to refer to the modular connector used with Ethernet twisted-pair cables.
Simple Figure
Ethernet Cable
|
v
+-------------+
| RJ45 |
| Connector |
+-------------+
|
v
Ethernet Port
Important
RJ45 is a connector, not a networking protocol or a network device.
41. Repeater
A repeater receives a signal and regenerates it so that it can travel farther.
Figure
Network Signal
|
v
Repeater
|
v
Regenerated Signal
Use
To extend the effective reach of a network segment.
42. Hub
A hub is a basic networking device that connects multiple devices.
When a hub receives a frame, it generally sends the signal out through its other ports.
Figure
PC1
|
|
PC2 --- HUB --- PC3
|
|
PC4
Important
A traditional hub does not intelligently select the intended destination port.
43. Switch
A network switch connects devices within a LAN and forwards Ethernet frames toward the appropriate port based on MAC addresses.
Figure
PC1
|
|
PC2 -- SWITCH -- PC3
|
|
PC4
Advantages over a traditional hub
More efficient forwarding
Reduces unnecessary traffic
Each port can provide a separate collision domain in traditional Ethernet
44. Router
A router connects different IP networks and forwards packets based on network-layer addressing and routing information.
Example
LAN 1
|
|
Router
|
|
Internet
|
|
LAN 2
Common Home Network
Mobile ─┐
Laptop ─┤
PC ─────┼── Wi-Fi Router ── Internet
TV ─────┘
45. Gateway
A gateway is a device or system that provides communication between different networks or systems, and can translate between different protocols or architectures when necessary.
Figure
Network A
|
|
Gateway
|
|
Network B
A default gateway in an IP network is commonly the router through which a device reaches destinations outside its local network.
46. Wi-Fi Card
A Wi-Fi card or wireless network adapter allows a device to connect to a wireless network.
It may be:
Built into a laptop
Built into a smartphone
Added through USB or another interface
Installed as an expansion card
Example
Laptop
|
Wi-Fi Card
))))
)))) Wireless
))))
Wi-Fi Router
47. Network Devices – Quick Comparison
| Device | Main Function |
|---|---|
| Modem | Provides signal conversion/access connectivity for certain communication systems |
| Ethernet Card | Connects device to Ethernet network |
| RJ45 | Connector commonly used for Ethernet twisted-pair cables |
| Repeater | Regenerates signals |
| Hub | Broadcasts incoming signals/frames to connected ports |
| Switch | Forwards LAN frames based on MAC addresses |
| Router | Connects IP networks and routes packets |
| Gateway | Connects/translates between different networks or systems |
| Wi-Fi Card | Provides wireless network connectivity |
48. Network Types
Networks can be classified according to their geographical or functional coverage.
Important types:
PAN
LAN
MAN
WAN
49. PAN – Personal Area Network
PAN stands for:
Personal Area Network
It connects devices around an individual.
Examples
Smartphone connected to wireless earbuds
Smartphone connected to smartwatch
Laptop connected to a personal device
Figure
Smartwatch
|
|
Earbuds --- Mobile --- Laptop
Characteristics
Very small range
Usually personal
Bluetooth and similar technologies are common
50. LAN – Local Area Network
LAN stands for:
Local Area Network
A LAN covers a relatively small geographical area such as:
Home
School
Office
Computer laboratory
Building
Figure
PC1 ─┐
PC2 ─┤
PC3 ─┼── SWITCH ── Server
PC4 ─┤
PC5 ─┘
Characteristics
Limited geographical area
High-speed communication
Usually privately managed
Common in schools and offices
51. MAN – Metropolitan Area Network
MAN stands for:
Metropolitan Area Network
It covers a larger area than a LAN, typically a city or metropolitan region.
Figure
Building A ─────┐
|
Building B ── MAN Network ── Building C
|
|
Building D
Example
A network connecting multiple offices of an organisation across a city.
52. WAN – Wide Area Network
WAN stands for:
Wide Area Network
It covers a very large geographical area.
It can connect:
Cities
States
Countries
Continents
The Internet is a prominent example of a global interconnected network using WAN technologies.
Figure
City A
|
+------ WAN ------+
|
City B |
| |
+------ WAN ------+
|
City C
53. PAN vs LAN vs MAN vs WAN
| PAN | LAN | MAN | WAN |
|---|---|---|---|
| Personal area | Local area | Metropolitan area | Wide area |
| Very small range | Small area | City/metropolitan area | Very large area |
| Personal devices | Home/school/office | Multiple locations in a city | Countries/continents |
| Example: phone + earbuds | School lab | City-wide organisation network | Internet/global network |
Easy Memory Trick
PAN → Person
LAN → Local
MAN → Metropolitan
WAN → Wide
54. Network Topology
Network topology refers to the physical or logical arrangement of devices and connections in a network.
The syllabus includes:
Bus
Star
Tree
55. Bus Topology
In a Bus topology, all devices share a common backbone cable.
Figure
Computer 1
|
|
====+========================+====
| | |
Computer 2 Computer 3 Computer 4
Common Backbone
Advantages
Simple structure
Requires less cable than some other topologies
Easy to understand
Disadvantages
Failure of backbone can affect the network
Troubleshooting can be difficult
Performance can decrease as network traffic increases
Adding devices may disturb the network
56. Star Topology
In a Star topology, all devices are connected to a central device, usually a switch.
Figure
PC1
|
|
PC2 -------- SWITCH -------- PC3
|
|
PC4
Advantages
Easy to install
Easy to manage
Failure of one cable generally affects only one connected device
Easy to add/remove devices
Good performance with switches
Disadvantage
If the central switch fails, communication among connected devices can be disrupted.
57. Tree Topology
A Tree topology has a hierarchical structure.
It can be viewed as a combination of star networks arranged in levels.
Figure
Core
|
+-------+-------+
| |
Switch A Switch B
/ \ / \
PC1 PC2 PC3 PC4
Advantages
Suitable for large networks
Easy hierarchical management
Can be expanded by adding branches
Disadvantages
Failure of an important upper-level device can affect a large portion of the network
More complex than a simple star
58. Topology Comparison
| Bus | Star | Tree |
|---|---|---|
| Common backbone | Central device | Hierarchical structure |
| Simple | Easy to manage | Suitable for larger networks |
| Backbone failure affects network | Central device failure affects connected network | Higher-level failure can affect branches |
| Less commonly used in modern switched LANs | Very common in Ethernet LANs | Used for hierarchical network design |
59. Network Protocol
A network protocol is a set of rules that defines how devices communicate over a network.
Protocols specify things such as:
How data is formatted
How data is transmitted
How devices identify each other
How errors are handled
How communication sessions work
60. Important Network Protocols
The syllabus includes:
HTTP
FTP
PPP
SMTP
TCP/IP
POP3
HTTPS
TELNET
VoIP
61. HTTP
HTTP stands for:
HyperText Transfer Protocol
It is an application-layer protocol used for communication between web clients and web servers.
Example
When a browser requests a web page:
Browser
|
HTTP Request
|
v
Web Server
|
HTTP Response
|
v
Browser
HTTP is commonly associated with web addresses beginning with:
http://
62. HTTPS
HTTPS stands for:
HyperText Transfer Protocol Secure
It is HTTP carried over a secure encrypted connection, typically using TLS.
Example
https://example.com
HTTP vs HTTPS
| HTTP | HTTPS |
|---|---|
| HyperText Transfer Protocol | HyperText Transfer Protocol Secure |
| Not encrypted by HTTP itself | Provides encryption through TLS |
| Less suitable for sensitive communication | Provides confidentiality and integrity protections when correctly configured |
Uses http:// | Uses https:// |
63. FTP
FTP stands for:
File Transfer Protocol
It is used to transfer files between computers over a network.
Uses
Uploading files to servers
Downloading files
Managing files on FTP servers
Structure
Client
|
| FTP
v
FTP Server
64. PPP
PPP stands for:
Point-to-Point Protocol
PPP is a data-link protocol designed for communication over a direct point-to-point link.
It can provide:
Encapsulation of network-layer protocols
Link establishment and configuration
Authentication mechanisms in supported configurations
65. SMTP
SMTP stands for:
Simple Mail Transfer Protocol
It is used for sending and transferring email.
Structure
Sender Mail Client
|
SMTP
|
v
Mail Server
|
SMTP
|
v
Receiver Mail Server
Important
SMTP is primarily associated with sending email.
66. POP3
POP3 stands for:
Post Office Protocol Version 3
It is used by email clients to retrieve email from a mail server.
Structure
Mail Server
|
POP3
|
v
Email Client
Easy Memory
SMTP → Send mail
POP3 → Retrieve mail
67. TCP/IP
TCP/IP stands for:
Transmission Control Protocol / Internet Protocol
TCP/IP is a suite of networking protocols used for communication across interconnected networks.
Simplified Model
Application
|
Transport → TCP
|
Internet → IP
|
Network Access
TCP
TCP provides reliable, ordered delivery of a byte stream between applications.
IP
IP provides addressing and routing of packets between networks.
68. TELNET
TELNET is a protocol used for remote text-based login/access to another computer over a network.
Important Security Point
Traditional Telnet does not encrypt the communication.
Therefore, it is generally not suitable for secure remote administration on modern networks.
SSH is commonly used instead when secure remote access is required.
Basic Structure
User Computer
|
TELNET
|
v
Remote Computer
69. VoIP
VoIP stands for:
Voice over Internet Protocol
VoIP allows voice communication to be carried over IP networks.
Example
Internet-based voice calling applications use IP networks to transmit voice.
Structure
Voice
|
v
Digital Data
|
v
IP Network
|
v
Digital Data
|
v
Voice
70. Protocol Summary Table
| Protocol | Full Form | Main Use |
|---|---|---|
| HTTP | HyperText Transfer Protocol | Web communication |
| HTTPS | HyperText Transfer Protocol Secure | Secure web communication |
| FTP | File Transfer Protocol | File transfer |
| PPP | Point-to-Point Protocol | Point-to-point data-link communication |
| SMTP | Simple Mail Transfer Protocol | Sending/transferring email |
| TCP/IP | Transmission Control Protocol / Internet Protocol | Internet/network communication |
| POP3 | Post Office Protocol Version 3 | Retrieving email |
| TELNET | Teletype Network | Remote text-based access |
| VoIP | Voice over Internet Protocol | Voice communication over IP networks |
71. Easy Protocol Memory Trick
HTTP → Web
HTTPS → Secure Web
FTP → Files
SMTP → Send Email
POP3 → Receive/Retrieve Email
TELNET → Remote Login
TCP/IP → Internet Communication
PPP → Point-to-Point
VoIP → Voice
72. Introduction to Web Services
The World Wide Web provides services through web technologies.
Important terms include:
WWW
HTML
XML
Domain Name
URL
Website
Web Browser
Web Server
Web Hosting
73. WWW
WWW stands for:
World Wide Web
The Web is a system of interlinked resources that are accessed over the Internet using web technologies and protocols such as HTTP/HTTPS.
Important Difference
Internet ≠ WWW
The Internet is the global network infrastructure.
The WWW is a service/system that operates over the Internet.
Example
Internet
|
+-- WWW
+-- Email
+-- VoIP
+-- File transfer
+-- Other Internet services
74. HTML
HTML stands for:
HyperText Markup Language
HTML is the standard markup language used to structure content on web pages.
Example
<html>
<head>
<title>My Page</title>
</head>
<body>
<h1>Welcome</h1>
<p>This is my web page.</p>
</body>
</html>
HTML is Used For
Headings
Paragraphs
Images
Links
Tables
Forms
Page structure
75. XML
XML stands for:
Extensible Markup Language
XML is a markup language designed to represent and transport structured data.
Example
<student>
<name>Rahul</name>
<roll>101</roll>
<marks>90</marks>
</student>
Important Difference
HTML mainly focuses on structuring/presenting web content, while XML is designed primarily to represent structured data.
76. HTML vs XML
| HTML | XML |
|---|---|
| HyperText Markup Language | Extensible Markup Language |
| Used to structure web pages | Used to represent structured data |
| Has predefined standard elements | Allows user-defined element names |
| Commonly rendered by browsers | Usually processed by applications |
| Focuses on document structure/presentation | Focuses on data structure and meaning |
77. Domain Name
A domain name is a human-readable name used to identify an Internet resource or website.
Example:
example.com
Instead of remembering a numerical IP address, users can use a domain name.
Example
Domain Name
|
v
example.com
|
v
DNS
|
v
IP Address
DNS translates domain names into IP addresses and other DNS records.
78. URL
URL stands for:
Uniform Resource Locator
A URL specifies the location of a resource and how it should be accessed.
Example
https://www.example.com/index.html
Main Parts
https://www.example.com/index.html
| | |
Protocol Domain Resource
A URL may also contain:
Port
Path
Query string
Fragment
Example:
https://example.com/products?id=10#details
79. Website
A website is a collection of related web pages and resources available under a common domain or web presence.
Examples of website resources may include:
Home page
About page
Contact page
Services page
Blog pages
Images
Videos
Forms
Structure
WEBSITE
|
+------------+------------+
| | |
Home About Contact
|
Services
|
Blog
80. Web Browser
A web browser is software used to access and display web resources.
Examples include:
Google Chrome
Mozilla Firefox
Microsoft Edge
Apple Safari
Browser Functions
A browser can:
Request web pages
Interpret HTML/CSS/JavaScript
Display web content
Download files
Store cookies and local data
Communicate with web servers
81. Web Server
A web server is a computer system and/or server software that stores and delivers web content to clients.
Structure
Browser
|
HTTP/HTTPS Request
|
v
Web Server
|
| Web Page/Data
v
Browser
A web server may serve:
HTML
CSS
JavaScript
Images
Videos
Other web resources
82. Web Hosting
Web hosting is a service that provides server resources and connectivity for making a website available on the Internet.
Basic Process
Website Files
|
v
Hosting Server
|
v
Domain Name
|
v
Internet
|
v
Visitors
Example
A developer creates:
index.html
about.html
contact.html
style.css
These files can be uploaded to a web hosting server.
Users can then access the website through its domain name.
83. Domain, Hosting and Website – Difference
| Term | Meaning |
|---|---|
| Domain Name | Human-readable Internet name |
| Web Hosting | Server resources where website files/services are hosted |
| Website | Collection of web pages/resources |
| Web Server | System/software that serves web resources |
| Browser | Client software used to access web resources |
| URL | Address/location of a specific resource |
84. How a Website Opens
Suppose the user enters:
https://www.example.com
Simplified Process
User
|
| Enters URL
v
Web Browser
|
| DNS lookup
v
IP Address
|
| HTTPS request
v
Web Server
|
| Response
v
Browser
|
v
Web Page Displayed
85. Complete Web Communication Structure
INTERNET
|
+--------------+--------------+
| |
Web Server DNS Server
| |
| |
+-------------+---------------+
|
Web Browser
|
v
User
86. Internet vs WWW
| Internet | WWW |
|---|---|
| Global network infrastructure | Web system/service running over Internet |
| Connects networks and devices | Provides linked web resources |
| Supports many services | One major Internet service |
| Includes email, VoIP, etc. | Primarily web pages and web applications |
Remember
Internet is the network; WWW is a service that uses the network.
87. Complete Unit 2 Concept Structure
COMPUTER NETWORKS
|
+---------------------+----------------------+
| | |
Evolution Data Communication Transmission Media
| | |
ARPANET Sender Wired
NSFNET Receiver |
Internet Message +-----+-----+
Medium | | |
Protocol Twisted Coaxial Fibre
Bandwidth Pair Cable Optic
IP Address
Switching
|
+-----+-----+
| |
Circuit Packet
Switching Switching
COMPUTER NETWORKS
|
+----------+----------+
| |
Network Devices Network Types
| |
+---------+---------+ +----+----+
| | | | | | | |
Hub Switch Router ... PAN LAN MAN WAN
|
Topologies
|
+-------+-------+
| | |
Bus Star Tree
COMPUTER NETWORKS
|
Protocols
|
+------+------+------+------+------+------+
| | | | | | |
HTTP FTP SMTP TCP/IP HTTPS POP3 TELNET
|
VoIP
|
Web Services
|
+--------+--------+--------+--------+
| | | | |
WWW HTML XML URL Domain
|
+------+------+
| |
Web Server Web Hosting
|
Web Browser
88. Important Differences for Examination
Hub vs Switch
| Hub | Switch |
|---|---|
| Basic multiport device | More intelligent LAN device |
| Sends incoming traffic to multiple ports | Forwards frames based on MAC addresses |
| Less efficient | More efficient |
| Traditional shared-collision environment | Each switch port is a separate collision domain in normal switched Ethernet |
Switch vs Router
| Switch | Router |
|---|---|
| Mainly connects devices within a LAN | Connects different IP networks |
| Uses MAC addresses for Ethernet forwarding | Uses IP addresses and routing information |
| Commonly works at Layer 2 | Primarily works at Layer 3 |
| Example: PCs connected within school LAN | Connects school LAN to Internet |
Repeater vs Hub
| Repeater | Hub |
|---|---|
| Regenerates/repeats signals | Connects multiple devices and repeats traffic to ports |
| Used to extend signal reach | Used to connect multiple network devices |
| Does not make routing decisions | Does not intelligently select destination |
HTTP vs HTTPS
HTTP → Web communication
HTTPS → Web communication + TLS security
SMTP vs POP3
SMTP → Sending/transferring email
POP3 → Retrieving email
Text vs Binary vs CSV
Text → Characters
Binary → Binary/object data
CSV → Rows and columns
89. Important Numerical Units
Networking commonly uses:
bit
|
bps
|
Kbps
|
Mbps
|
Gbps
|
Tbps
Remember:
b= bitB= byte
Therefore:
Mbps ≠ MBps
A byte contains 8 bits.
90. Important One-Line Definitions
Computer Network
A group of interconnected computing devices that communicate and share resources.
Data Communication
Exchange of data between devices through a communication medium.
Protocol
A set of rules governing communication between devices.
Bandwidth
The capacity of a communication channel to carry data, commonly expressed in bits per second.
IP Address
An address used to identify a network interface/device within an IP network.
Circuit Switching
Communication using a dedicated path established for a session.
Packet Switching
Communication in which data is divided into packets that are transmitted through a shared network.
Repeater
A device that regenerates signals to extend network reach.
Hub
A basic device that repeats incoming network traffic to its ports.
Switch
A LAN device that forwards Ethernet frames based on MAC addresses.
Router
A device that forwards IP packets between networks.
Gateway
A device/system that connects different networks or systems and may perform protocol translation.
LAN
A network covering a relatively small geographical area.
WAN
A network covering a large geographical area.
Topology
The arrangement of devices and links in a network.
HTTP
Protocol used for web communication.
FTP
Protocol used for file transfer.
SMTP
Protocol used for sending/transferring email.
POP3
Protocol used for retrieving email from a mail server.
HTTPS
HTTP secured using TLS.
VoIP
Transmission of voice communication over IP networks.
WWW
A system of interlinked web resources accessed over the Internet.
HTML
Markup language used to structure web pages.
XML
Markup language used to represent structured data.
URL
Uniform Resource Locator; identifies the location/access method of a resource.
Domain Name
Human-readable name used to identify an Internet resource.
Web Browser
Software used to access and display web resources.
Web Server
A system/software that serves web resources to clients.
Web Hosting
Service that provides server resources for making websites/services available online.
91. Important Board Examination Questions
Very Short Answer Questions
What is a computer network?
What is ARPANET?
What is NSFNET?
What is the Internet?
What is data communication?
Name the five components of data communication.
What is a protocol?
What is bandwidth?
What is an IP address?
What is packet switching?
What is circuit switching?
What is a repeater?
What is a hub?
What is a switch?
What is a router?
What is RJ45?
What is PAN?
What is LAN?
What is MAN?
What is WAN?
What is network topology?
What is HTTP?
What is FTP?
What is SMTP?
What is POP3?
What is HTTPS?
What is VoIP?
What is WWW?
What is HTML?
What is XML?
What is a URL?
What is a domain name?
What is web hosting?
92. Important Short-Answer Questions
Q1. Explain the components of data communication.
Answer:
The five main components are:
Sender – sends the data.
Receiver – receives the data.
Message – information being transmitted.
Communication Medium – path through which data travels.
Protocol – rules governing communication.
Q2. Differentiate between circuit switching and packet switching.
Answer:
Circuit switching establishes a dedicated path before communication, whereas packet switching divides data into packets and sends them through a shared network.
Q3. Explain PAN, LAN, MAN and WAN.
Answer:
PAN – personal area network.
LAN – local area network.
MAN – metropolitan area network.
WAN – wide area network.
They mainly differ in geographical coverage and scale.
Q4. Explain Bus, Star and Tree topology.
Answer:
Bus – all devices share a common backbone.
Star – all devices connect to a central device.
Tree – devices are arranged in a hierarchical structure.
Q5. Differentiate between a hub, switch and router.
Answer:
A hub repeats traffic to connected ports, a switch forwards Ethernet frames within a LAN using MAC addresses, and a router forwards IP packets between different networks using IP addressing and routing information.
Q6. Differentiate between HTTP and HTTPS.
Answer:
HTTP is used for web communication. HTTPS is HTTP protected by TLS, providing encryption and integrity protection for the connection.
Q7. Differentiate between SMTP and POP3.
Answer:
SMTP is used primarily for sending/transferring email, whereas POP3 is used by a client to retrieve email from a mail server.
Q8. Differentiate between HTML and XML.
Answer:
HTML is primarily used to structure web-page content, while XML is designed to represent structured data using user-defined tags.
93. Important Application-Based Questions
Question 1
A school has 40 computers connected inside one computer laboratory.
Which type of network is suitable?
Answer: LAN.
Question 2
A student connects a smartwatch to a smartphone.
Which type of network is this?
Answer: PAN.
Question 3
An organisation connects offices located in different cities.
Which type of network is suitable?
Answer: WAN.
Question 4
Which device should be used to connect different IP networks?
Answer: Router.
Question 5
Which device regenerates a weak network signal?
Answer: Repeater.
Question 6
Which device forwards Ethernet frames using MAC addresses?
Answer: Switch.
Question 7
Which protocol is commonly used for secure web communication?
Answer: HTTPS.
Question 8
Which protocol is primarily used for sending email?
Answer: SMTP.
Question 9
Which protocol is used to retrieve email using the traditional POP mechanism?
Answer: POP3.
Question 10
Which technology is used to transmit data through light?
Answer: Fibre-optic cable.
94. Final Quick Revision Chart
COMPUTER NETWORKS
|
+-- Evolution
| |
| ARPANET → NSFNET → Internet
|
+-- Data Communication
| |
| Sender
| Receiver
| Message
| Medium
| Protocol
|
+-- Transmission Media
| |
| Wired
| ├─ Twisted Pair
| ├─ Coaxial
| └─ Fibre Optic
|
| Wireless
| ├─ Radio
| ├─ Microwave
| └─ Infrared
|
+-- Devices
| |
| Modem
| Ethernet Card
| RJ45
| Repeater
| Hub
| Switch
| Router
| Gateway
| Wi-Fi Card
|
+-- Network Types
| |
| PAN
| LAN
| MAN
| WAN
|
+-- Topologies
| |
| Bus
| Star
| Tree
|
+-- Protocols
| |
| HTTP
| FTP
| PPP
| SMTP
| TCP/IP
| POP3
| HTTPS
| TELNET
| VoIP
|
+-- Web Services
|
WWW
HTML
XML
Domain Name
URL
Website
Web Browser
Web Server
Web Hosting
95. Golden Rules for Class 12
ARPANET → Early network development
NSFNET → Academic/research network expansion
Internet → Global network of networks
PAN → Personal
LAN → Local
MAN → Metropolitan
WAN → Wide
Bus → Backbone
Star → Central device
Tree → Hierarchical
Hub → Repeats traffic
Switch → MAC-based LAN forwarding
Router → Connects/routes between IP networks
Gateway → Connects/translates different networks/systems
Repeater → Regenerates signal
HTTP → Web
HTTPS → Secure Web
FTP → Files
SMTP → Send Email
POP3 → Retrieve Email
TELNET → Remote text-based access
VoIP → Voice over IP
HTML → Web-page structure
XML → Structured data
URL → Resource locator
DNS → Domain-name resolution
Browser → Accesses web resources
Server → Serves web resources
Hosting → Provides server resources for websites
CONCLUSION
Computer networking is the foundation of modern digital communication. A good understanding of network evolution, data communication, transmission media, network devices, network types, topologies, protocols and web technologies is essential for Class 12 Computer Science.
For CBSE examination preparation, students should especially remember:
ARPANET → NSFNET → Internet
Sender + Receiver + Message + Medium + Protocol
Bandwidth + Data Transfer Rate + IP Address
Circuit Switching vs Packet Switching
Twisted Pair + Coaxial + Fibre Optic
Radio + Microwave + Infrared
Modem + Repeater + Hub + Switch + Router + Gateway
PAN + LAN + MAN + WAN
Bus + Star + Tree
HTTP + FTP + PPP + SMTP + TCP/IP + POP3 + HTTPS + TELNET + VoIP
WWW + HTML + XML + Domain Name + URL + Website + Browser + Web Server + Web Hosting
These are the core concepts of CBSE Class 12 Computer Science (083) – Unit 2: Computer Networks.
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