Class 9 Computer Science Notes Chapter 6

Chapter 6: Introduction to Computer Networks

Exercise Solution

PART I: MULTIPLE-CHOICE QUESTIONS (MCQs)

Q1. What is the primary objective of computer networks?

(a) Increase computational power
(b) Enable resource sharing and data communication
(c) Enhance graphic capabilities
(d) Improve software development

Correct Answer: (b) Enable resource sharing and data communication

Q2. Which device is used to connect multiple networks and direct data packets between them?

(a) Switch
(b) Hub
(c) Router
(d) Modem

Correct Answer: (c) Router

Q3. Which layer of the OSI model is responsible for node-to-node data transfer and error detection?

(a) Physical Layer
(b) Data Link Layer
(c) Network Layer
(d) Transport Layer

Correct Answer: (b) Data Link Layer

Q4. What is the function of the Domain Name System (DNS)?

(a) Assign IP addresses dynamically
(b) Translate domain names to IP addresses
(c) Secure data communication
(d) Monitor network traffic

Correct Answer: (b) Translate domain names to IP addresses

Q5. Which method of data transmission uses a dedicated communication path?

(a) Packet Switching
(b) Circuit Switching
(c) Full-Duplex
(d) Half-Duplex

Correct Answer: (b) Circuit Switching

Q6. What is encapsulation in the context of network communication?

(a) Converting data into a secure format
(b) Wrapping data with protocol information
(c) Monitoring network traffic
(d) Translating domain names to IP addresses

Correct Answer: (b) Wrapping data with protocol information

Q7. Which protocol is used for reliable data transfer in the TCP/IP model?

(a) HTTP
(b) FTP
(c) TCP
(d) UDP

Correct Answer: (c) TCP

Q8. What is the main purpose of a firewall in network security?

(a) Convert data into a secure format
(b) Monitor and control network traffic
(c) Assign IP addresses
(d) Translate domain names

Correct Answer: (b) Monitor and control network traffic

Q9. Which network topology connects all devices to a central hub?

(a) Ring
(b) Mesh
(c) Bus
(d) Star

Correct Answer: (d) Star

PART II: SHORT QUESTIONS

Q1. Define data communication and list its key components.

Answer: Data communication refers to the exchange of data (digital information) between two or more devices over a transmission medium. Its five key components are:

Message: The data or information to be communicated.
Sender: The device that generates and sends the data message.
Receiver: The device that receives the data message.
Transmission Medium: The physical path, such as coaxial cable, fiber optics, or wireless airwaves, connecting the sender and receiver.
Protocols: A set of rules governing data transmission.

Q2. Explain the role of routers in a computer network.

Answer: Routers are networking devices that examine destination IP addresses attached to incoming data packets, determine the most efficient communication path across interconnected networks, and forward the packets toward their final destination.

Q3. What are the main functions of the Network Layer in the OSI model?

Answer: The Network Layer (Layer 3) is responsible for logical addressing, such as assigning IP addresses, packet routing across multiple interconnected networks, and managing congestion control.

Q4. Describe the difference between packet switching and circuit switching.

Answer:

Circuit Switching: Establishes a dedicated, continuous physical communication path between the sender and receiver for the entire duration of the session, such as traditional telephone lines.
Packet Switching: Breaks messages into smaller data packets that are routed independently across shared network pathways based on traffic conditions and reassembled at the destination.

Q5. What is the purpose of the Dynamic Host Configuration Protocol (DHCP)?

Answer: DHCP is a network management protocol used to automatically assign IP addresses, subnet masks, default gateways, and DNS configuration parameters to devices joining a network, eliminating manual configuration.

Q6. How does encapsulation ensure organized communication in a network?

Answer: Encapsulation wraps data payloads with specific protocol headers and trailers, such as source and destination addresses, sequence numbers, and error-checking codes, as data moves down the OSI layers. This ensures each layer processes and routes the information correctly.

Q7. Differentiate between TCP and UDP in terms of data transfer reliability.

Answer:

TCP (Transmission Control Protocol): A connection-oriented protocol that guarantees reliable delivery through acknowledgments, flow control, and retransmission of lost packets.
UDP (User Datagram Protocol): A connectionless protocol that offers fast, best-effort transmission without checking for packet delivery or guaranteeing sequencing. It is suitable for real-time streaming and gaming.

Q8. Explain the importance of encryption in network security.

Answer: Encryption transforms plain, readable data into unreadable ciphertext using cryptographic algorithms and secret keys. It ensures confidentiality by preventing unauthorized interceptors or hackers from reading sensitive data transmitted across public networks.

Q9. What are the advantages of using a star topology in a network?

Answer:

Easy installation and configuration.
If a single cable or peripheral fails, only that device is disconnected, leaving the rest of the network fully operational.
Centralized management and troubleshooting through the central hub or switch.

Q10. How do firewalls contribute to network security?

Answer: Firewalls act as a secure barrier between trusted internal networks and untrusted external networks, such as the Internet. They monitor, filter, and block incoming and outgoing network traffic based on predetermined security rules.

PART III: LONG QUESTIONS

Q1. Discuss the objectives of computer networks and provide examples of how they facilitate resource sharing and data communication.

Answer:

Primary Objectives:

Resource sharing
High reliability
Scalability
Cost reduction
Fast communication

Resource Sharing Examples:

Hardware Sharing: Multiple office computers connected to a single networked printer or central storage NAS drive.
Software Sharing: Enterprise applications hosted on central servers accessible across remote workstations.

Data Communication Examples:

Instant messaging
Corporate email exchange
Cloud file collaboration
Remote video conferencing

Q2. In a Simplex communication system, assume data is transmitted at a rate of 500 bits per second (bps). Compute the time to transmit a message if:

(a) It is of 10 kilobits.

Solution:

Total bits = 10 kilobits = 10,000 bits

Formula:

[
\text{Time}=\frac{\text{Total Bits}}{\text{Transmission Rate}}
]

[
\text{Time}=\frac{10,000}{500}=20\text{ seconds}
]

Answer: 20 seconds

(b) It is of 10 kilobytes.

Solution:

Total bytes = 10 kilobytes = (10 \times 1,024 = 10,240) bytes

Total bits = 10,240 bytes × 8 bits/byte

Total bits = 81,920 bits

[
\text{Time}=\frac{81,920}{500}=163.84\text{ seconds}
]

Answer: 163.84 seconds

Q3. Describe how data is transmitted across computer networks using packet switching and circuit switching.

Answer:

Circuit Switching: Before communication begins, a dedicated physical circuit is reserved between the sender and receiver. All data flows continuously along this fixed route until the connection terminates. If lines are busy, the call is blocked.
Packet Switching: Messages are divided into smaller chunks called packets. Each packet contains header data, including source and destination addresses. Packets travel independently over available network links, taking different routes if necessary, and are reassembled in the correct order at the destination. This maximizes network bandwidth utilization.

Q4. Discuss the role and importance of protocols in data communication. Explain the functions of key protocols such as TCP/IP, HTTP, DNS, and DHCP.

Answer:

Role of Protocols: Protocols provide standardized rules, syntax, and synchronization standards that allow diverse hardware and software made by different manufacturers to communicate seamlessly.

Key Protocols:

TCP/IP: The foundational communication suite ensuring reliable end-to-end packet delivery through TCP and logical Internet routing through IP.
HTTP (HyperText Transfer Protocol): Facilitates the transfer of web pages and multimedia files across the World Wide Web between web browsers and servers.
DNS (Domain Name System): Resolves human-readable domain names, such as www.example.com, into numerical IP addresses.
DHCP (Dynamic Host Configuration Protocol): Automatically assigns temporary IP addresses to devices connecting to a network.

Q5. Evaluate different methods of network security, including firewalls, encryption, and antivirus software.

Answer:

Firewalls: Inspect network traffic packets against security rules to block unauthorized external access. They are essential for perimeter defense.
Encryption: Secures data at rest and in transit so that intercepted information remains unreadable ciphertext without decryption keys. It is vital for e-commerce and privacy.
Antivirus Software: Scans system files and incoming downloads for malicious signatures to quarantine and remove malware infections. It is critical for endpoint protection.

Q6. Describe real-world applications of computer networks in business, education, and healthcare.

Answer:

Business: Enables e-commerce platforms, cloud-based financial accounting, supply chain management, and remote employee collaboration.
Education: Powers virtual classrooms, online learning management systems (LMS), digital research libraries, and collaborative student projects.
Healthcare: Facilitates Electronic Health Records (EHR), telemedicine consultations, real-time patient monitoring systems, and secure transmission of medical imaging data.

Q7. Compare and contrast the different types of network topologies (star, ring, bus, and mesh).

Answer:

Star Topology: All nodes connect to a central hub or switch. It is easy to manage, but failure of the central hub brings down the network.
Ring Topology: Devices are connected in a circular loop where data travels in one direction. Failure of one cable breaks the ring.
Bus Topology: Devices share a single central communication cable called a backbone. It is simple and inexpensive but vulnerable to cable breaks.
Mesh Topology: Every device has dedicated point-to-point links to multiple other nodes. It is highly redundant and reliable but expensive and complex to wire.

Q8. Consider a shift cipher with a shift amount of 4.

(a) Encrypt the message “SECURITY”.

Solution:

Applying a +4 shift to each letter (A = 0 to Z = 25):

S (18) + 4 = 22 → W
E (4) + 4 = 8 → I
C (2) + 4 = 6 → G
U (20) + 4 = 24 → Y
R (17) + 4 = 21 → V
I (8) + 4 = 12 → M
T (19) + 4 = 23 → X
Y (24) + 4 = 28 → 2 (mod 26) → C

Encrypted Message: WIGVYMXC

(b) Decrypt the message “WMXYVMI”.

Solution:

Applying a −4 shift to each letter:

W (22) − 4 = 18 → S
M (12) − 4 = 8 → I
X (23) − 4 = 19 → T
Y (24) − 4 = 20 → U
V (21) − 4 = 17 → R
M (12) − 4 = 8 → I
I (8) − 4 = 4 → E

Decrypted Message: SITURIE

Q9. An IPv4 address is a 32-bit number. Calculate the total number of unique IPv4 addresses possible.

(a) Show the calculation for the total number of IPv4 addresses.

Solution:

Total addresses = (2^{32})

[
2^{32}=4,294,967,296
]

Answer: 4,294,967,296 (approximately 4.29 billion addresses)

(b) How many addresses are left if 10% of the total addresses are reserved for special purposes?

Solution:

Reserved addresses = 10% of 4,294,967,296

Reserved addresses = 429,496,729.6

Remaining addresses = 90% of 4,294,967,296

Remaining addresses = 3,865,470,566.4

Answer: Approximately 3,865,470,566 addresses

Notes by Sir Dilawar

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