৪৯তম বিসিএস ⎯ কম্পিউটার সায়েন্স (CSE) [৯৭১]

Answer: A) Sharing resources
A computer network primarily enables devices to share resources such as data, software, and hardware equipment.

 Answer: D) VPN
Explanation:
• LAN (Local Area Network): small area (office, home).
• MAN (Metropolitan Area Network): city-level.
• WAN (Wide Area Network): global (Internet).
• VPN (Virtual Private Network): a secure overlay network, not a physical type of network.

Answer B
Intranet is secured and private → only employees or authorized users can log in (username & password).

It often works over LAN/WAN and is accessed via web browsers, just like the Internet.

Answer D) Hub

Router → Connects multiple networks (e.g., home LAN to Internet) and forwards data packets.

Repeater → Regenerates and amplifies signals to extend network range.

Modem → Converts digital signals ↔ analog signals for Internet over telephone/cable lines.

Hub → Simple device is used to connect multiple nodes of network that broadcasts data to all connected devices in a LAN.

 Answer: B) Dynamic Host Configuration Protocol

DHCP (Dynamic Host Configuration Protocol) is a network management protocol used to automatically assign IP addresses and other configuration details (subnet mask, gateway, DNS) to devices on a network.

Without DHCP, admins would have to manually configure each device’s IP.

Example: When you connect your laptop or phone to Wi-Fi, the DHCP server gives it an IP automatically.

Answer D
Communication mode: Radio/TV broadcasting is one-way communication → sender transmits, receivers only listen.

This is called Simplex mode.

Example: A radio station sends signals; you can only listen, not talk back.

Type of transmission:  Broadcast means sending data to all devices in a network/area.

Radio/TV signals are sent to all receivers in range, not to a single device (unicast) or group (multicast).

Why not others?

Unicast → one-to-one →since broadcast is one-to-many).
Multicast → one-to-selected group
Full duplex/Half duplex → imply two-way communication →  (radio/TV is only one-way).

Answer C) Timing Control
Bit synchronization means the sender and receiver must agree on when each bit starts and ends during transmission.

Without synchronization, the receiver may misinterpret bit boundaries (e.g., reading 1011 as 1101).

Therefore, it’s fundamentally a timing control issue.

Timing Control → Ensures that the receiver samples incoming signals at the right moment (middle of bit interval).

How achieved? -Using clock signals (Synchronous transmission).

Start/Stop bits or special patterns (Asynchronous transmission).

Why not others?

Transmission Control (a): About controlling data sending, not bit timing.
Reception Control (c): Deals with handling received data, not bit timing.
Data Control (d): Involves data flow/error handling, not synchronization. 

Answer D
wisted Pair Cable (UTP/STP): Most commonly used in Ethernet LANs.

Pairs of wires are twisted to reduce electromagnetic interference (EMI).

Examples: Cat5e, Cat6, Cat6a cables.

Coaxial cable: Used in older Ethernet networks (10BASE2, 10BASE5), less common today.

Fiber Optic: High-speed, long-distance, expensive → used for backbone or special applications.

HDMI: Video/audio cable, not for networking.

Ethernet standards:

Cat5e: up to 1 Gbps
Cat6: up to 10 Gbps (short distances)
Fiber: 10 Gbps+

Answer -  B)
Server is used to allocate and manage resources for a network.

Answer: C
Full Mesh Topology: In a full mesh topology, each computer is connected to all the computers available in the network.

Features of Mesh Topology
There are multiple paths from one computer to another computer.
It does not contain the switch, hub or any central computer which acts as a central point of communication.
Mesh topology is mainly used for wireless networks.
Mesh topology can be formed by using the formula:
Number of cables = (n*(n-1))/2;

Answer: D) HTTP

HTTP (Hypertext Transfer Protocol): Primary protocol used for transmitting web pages and other content from a web server to a web browser.
Works on the Application Layer of the OSI model.
Uses TCP port 80 (HTTPS uses 443 for secure transfer).

SMTP (Simple Mail Transfer Protocol): Used for sending emails. 
FTP (File Transfer Protocol): Used for transferring files between computers. 
SNMP (Simple Network Management Protocol): Used for network management and monitoring. 


Web pages → HTTP/HTTPS
Emails → SMTP/POP3/IMAP
Files → FTP/SFTP
Network monitoring → SNMP

Answer: A) Open Systems Interconnection
OSI is a reference model that standardizes network communication into 7 layers

 Answer: B) To provide data routing paths for network communication
Network Layer Function: Operates at Layer 3 of the OSI model.

Responsible for logical addressing (IP addresses) and determining the best path for data packets to travel from the source host to the destination host, even across multiple networks.

Key Tasks:

Routing: Determines the path for packets (using routers and routing protocols).

Logical Addressing: Assigns IP addresses for devices on the network.

Packet Forwarding: Moves packets toward the destination.

A) Format data: This is done by the Presentation layer (Layer 6).
C) Establish connections: This is the responsibility of the Transport or Session layer.
D) Encode and decode data: Also done by the Presentation layer. 

NOTE:
Network Layer = Routing + Logical Addressing

Data Link Layer = Node-to-node delivery + MAC addresses

Physical Layer = Bits → Signals

Answer: C) Application
Explanation: TCP/IP combines the top three OSI layers into Application layer, simplifying the model.

Answer: C) To detect and manage when two devices try to send data simultaneously

Explanation

Collision detection (CSMA/CD) is used in Ethernet networks to detect and manage when two devices attempt to send data over the network at the same time.

Answer: B) TCP

TCP (Transmission Control Protocol) ensures reliable, sequenced delivery, flow control, and error correction.

UDP is connectionless → unreliable.

IP operates at Network/Internet layer

Answer: Network Access/Interface layer

The Network access/Interface layer of the TCP/IP model corresponds to both the Physical and Data Link layers of the OSI model, dealing with physical connectivity and data framing.

Answer: B
Explanation: Repeaters regenerate/boost signals at the physical layer (Layer 1).
Switches/Bridges are Layer 2
Routers are Layer 3

Answer: C
Explanation: Logical (IP) addressing is Layer 3.i.e Data link layer
Framing, error detection, and MAC are Layer 2.

Answer: D Physical

Connectivity issues typically relate to the Physical layer, involving hardware components like cables, NICs, and switches/routers.

Answer: C ) To route packets across networks

The Internet Protocol (IP) is designed to route packets across networks, enabling internetworking and essentially establishing the Internet.

Answer B) WWW
The collection of the hyperlinked document on the internet known as WWW (World Wide Web)

Answer: A) Address size

Explanation: The main difference between IPv4 and IPv6 is the address size, with IPv6 having 128-bit addresses compared to IPv4's 32-bit addresses, significantly increasing the available address space.

Answer: C
Explanation: IPv6 = 128-bit, written in hex colon notation

Answer A
A subnet mask is a 32-bit number used in IPv4 networking that helps determine which part of an IP address identifies the network and which part identifies the host within that network.

It looks similar to an IP address, for example:

IP address:    192.168.1.10
Subnet mask:   255.255.255.0

2. How it works

The network portion is indicated by the 1s in the subnet mask.

The host portion is indicated by the 0s.

For the example above:

Subnet mask: 255.255.255.0
Binary:      11111111.11111111.11111111.00000000

The first 24 bits (11111111.11111111.11111111) represent the network.

The last 8 bits (00000000) represent the host.

So the IP 192.168.1.10 is in the network 192.168.1.0, and the host identifier is 10.

3. Why not the other options?

B. To encrypt IP addresses:  Subnet masks have nothing to do with encryption.

C. To detect IP conflicts:  Conflicts are detected by protocols like ARP, not by the subnet mask.

D. To assign IP addresses automatically: that’s the job of DHCP, not a subnet mask.

Answer: A


Class A Default Subnet Mask
In IPv4, IP addresses are divided into classes (A, B, C, etc.) mainly for addressing and subnetting.
An IP address has two parts:

Network ID → identifies the network.
Host ID → identifies hosts inside that network.
 
Class A Address Range
First octet: 1 – 127 (127 is reserved for loopback).

Format: N.H.H.H

N = Network portion (first octet).
H = Host portion (last three octets).
 
Default Subnet Mask
For Class A, only the first octet (8 bits) is reserved for the network.
The remaining 24 bits are used for hosts.
So the default subnet mask is:

 11111111.00000000.00000000.00000000
 255.0.0.0

Answer: C

Answer: D

private IP addresses:
Private IPv4 addresses are IP addresses reserved for use within private networks. They cannot be routed on the public Internet, meaning devices using these addresses must use NAT (Network Address Translation) to communicate with the Internet.

These addresses are defined by RFC 1918

Answer: A
Explanation: 127/8 reserved for loopback.

A loopback address is a special IP address used to test the network stack of a local device.
• It allows a device to send packets to itself without using the network hardware.
• It’s mainly used for diagnosing TCP/IP software and network applications.

2. IPv4 Loopback Range
• The entire range 127.0.0.0 – 127.255.255.255 is reserved for loopback purposes.
• The most commonly used address is 127.0.0.1, often called localhost.

• B) 255.255.255.255 -This is the broadcast address for sending packets to all hosts on a local network.
• C) 0.0.0.0 --Represents “any IP address” and is used for default routing, not loopback.
• D) 169.254.0.1 - Part of APIPA (Automatic Private IP Addressing) range (169.254.0.0/16) for automatic addressing when DHCP fails.
________________________________________
4. Summary Table
Feature          IP Address / Range
Loopback      127.0.0.0 – 127.255.255.255
Common loopback    127.0.0.1 (localhost)
Purpose Test local TCP/IP stack without network hardware.

Classful IPv4 Addressing

Class A → 1.0.0.0 –     126.255.255.255 (default mask: 255.0.0.0)

Class B → 128.0.0.0 – 191.255.255.255 (default mask: 255.255.0.0)

Class C → 192.0.0.0 – 223.255.255.255 (default mask: 255.255.255.0)

Class D → 224.0.0.0 – 239.255.255.255 (Multicast)

Class E → 240.0.0.0 – 255.255.255.255 (Experimental / Research)

Step-by-Step Check

A) 172.16.5.4 → First octet = 172 → falls in Class B (128–191).

B) 192.168.1.1 → First octet = 192 → falls in Class C (192–223).

C) 10.0.0.1 → First octet = 10 → falls in Class A (1–127).

D) 224.5.6.7 → First octet = 224 → falls in Class D (224–239).

So, Answer: B) 192.168.1.1

Answer: B) RIP 

Distance Vector Routing is a type of dynamic routing protocol where each router periodically shares its entire routing table with its direct neighbors.

"Distance" = Cost metric (e.g., hop count, delay).

"Vector" = Direction (next-hop router).

Examples

RIP (Routing Information Protocol) – uses hop count (max = 15).

IGRP (Interior Gateway Routing Protocol) – Cisco proprietary.

Answer: C) From one sender to exactly one receiver 
Explanation:
Unicast = one-to-one communication.

Answer: D) Congestion window 
Explanation: TCP uses Congestion window (cwnd) and algorithms like slow start, AIMD to manage congestion

What is congestion?

Congestion happens when too much data is injected into the network, and routers/switches can’t process it fast enough → packet drops, delays, retransmissions.

TCP must detect and handle congestion to avoid network collapse.

2. TCP’s Key Features

Checksum → Error detection (corruption check).  Not for congestion control.

Sliding Window → Controls how many bytes can be in transit → ensures reliability.  More about flow & sequencing.

Flow Control → Uses receiver window (rwnd) to prevent sender from overwhelming receiver.  It’s receiver-based, not network congestion-based.

Congestion Control → Uses congestion window (cwnd) to limit how much data is sent into the network →  this manages congestion.

Answer: C) To determine optimal data paths

Routing = process of finding the best path for data packets to travel from a source to a destination across interconnected networks.

2. What are Routing Protocols?

Routing protocols are rules and algorithms that routers use to exchange information and automatically learn/update routes.

They don’t carry user data, instead they carry routing information.

"Optimal" = best path based on metrics (hop count, bandwidth, delay, reliability, cost, etc.).

Example:

RIP → chooses path with least hops.

OSPF → chooses path with lowest cost (based on bandwidth).

BGP → chooses path based on policies and AS-paths.

 Answer: B) Filtering traffic between trusted and untrusted networks

A firewall acts as a security barrier between a trusted internal network (like your LAN) and an untrusted external network (like the Internet).
Its primary job is to allow or block network packets based on a set of rules (IP address, port, protocol, etc.).

Firewalls can be hardware-based, software-based, or both.

Why not the other options?

A) Encrypting data → That’s the role of technologies like SSL/TLS or VPNs, not the firewall’s main function.

C) Storing user passwords → That’s done by authentication systems, not firewalls.

D) Increasing bandwidth → A firewall doesn’t increase speed; in fact, filtering can slightly reduce throughput.

A firewall’s core function = Traffic Filtering & Access Control

Answer B

1. Streaming Media Characteristics

Needs fast, continuous delivery (video/audio frames).
Tolerates some packet loss (a missing frame in a video is okay). does not tolerate delay (late packets are useless).
So → protocols used for streaming must be low-latency and not waste time on retransmissions.

TCP (Transmission Control Protocol): Reliable, connection-oriented.
Retransmits lost packets → introduces delay (bad for real-time).
Used in web browsing, emails, file transfers.

UDP (User Datagram Protocol) : Connectionless, lightweight.
No retransmission = lower latency.
Widely used for streaming (VoIP, IPTV, online gaming, live video).

SCTP (Stream Control Transmission Protocol) : Used in telecom signaling, supports multi-streaming.
Not common for media streaming.
RTP (Real-time Transport Protocol)
RTP itself rides on top of UDP.

Provides sequence numbering, timestamps (important for media synchronization).

Used in VoIP, video conferencing, live streaming.

Answer: B) TCP is connection-oriented and reliable, UDP is connectionless and unreliable 

Explanation:

TCP (Transmission Control Protocol):

1. Establishes a connection before sending data (connection-oriented).
2. Guarantees delivery using ACKs, sequencing, and retransmissions (reliable).
3. Ensures data arrives in order.

UDP (User Datagram Protocol):

1. Sends data without establishing a connection (connectionless).
2. Does not guarantee delivery (unreliable).
3. No ordering, no flow/congestion control → faster, lightweight.

Answer: C) To synchronize sequence numbers and acknowledge the connection 

1. What is TCP Three-Way Handshake?

TCP is a connection-oriented protocol, which means a reliable connection must be established between the sender and receiver before any actual data transfer.

The three-way handshake is the process TCP uses to:

→ Establish the connection
→ Synchronize sequence numbers
→ Confirm both sides are ready to send/receive data

Steps of the Handshake

SYN (Synchronize): Client sends a segment with the SYN flag set and an initial sequence number (ISN) to the server.
SYN-ACK (Synchronize-Acknowledge): Server responds with its own SYN and acknowledges the client’s ISN.
ACK (Acknowledge): Client sends an ACK to confirm the server’s sequence number.

After this, the connection is established and data can be transmitted reliably.

The TCP three-way handshake ensures:

→ Both sides agree on starting sequence numbers
→ Both sides are ready to communicate reliably

Answer: B
Explanation:
802.11 = WLAN (Wifi); 
802.3 = Ethernet; 
802.16 = WiMAX.
802.15= Bluetooth 

Answer: A
VSAT = Very Small Aperture Terminal/ small satellite earth stations.

Answer B

POP3 (Post Office Protocol version 3): Downloads emails from the server to your local device. Emails can be deleted from the server after download (configurable). Simpler, widely used.

IMAP (Internet Message Access Protocol): Access emails directly on the server without downloading. Allows synchronization across multiple devices.

SMTP (Simple Mail Transfer Protocol): Used to send emails from client to server or between servers. Does NOT retrieve emails. FTP / HTTP

Not used for email retrieval. FTP is for file transfer, HTTP is for web pages.

Answer D

Explanation: ATM uses virtual circuits (VC) to provide predictable QoS for real-time traffic.

Answer: B
Explanation: Hosts use IGMP to join/leave multicast groups.

Answer: C
Explanation: DoS modifies/interrupts; eavesdropping is passive.

Answer: B
CA = trusted third party.

Answer D

Explanation: VPN encrypts traffic over any network, including wireless networks, protecting data from interception.

Answer B

Explanation: ATM switches operate at Layer 2 (Data Link) using virtual circuits for switching fixed-size cells.

Answer: C) Both ipconfig and ifconfig

1.    ipconfig
• Used in Windows OS.
• Displays IP address, subnet mask, default gateway, and other network settings.
• Example: ipconfig /all shows detailed information.

2.    ifconfig
• Used in Linux/Unix/Mac OS.
• Shows network interface configuration (IP, netmask, broadcast address).
• Can also enable/disable interfaces.
 netstat?
 netstat shows active connections, listening ports, routing table → doesn’t show full IP configuration.