2020 question paper

Computer Networks

23 questions

Q1a. An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: 245.248.128.0/20 The ISP wants to give half of this chunk of addresses to organization A, and a quarter to organization B, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to A and B? (i) 245.248.136.0/21 and 245.248.128.0/22 (ii) 245.248.128.0/21 and 245.248.128.0/22 (iii) 245.248.132.0/22 and 245.248.132.0/21 (iv) 245.248.136.0/22 and 245.248.132.0/2120202m
Module 3: Network Layer & Routing
An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it:

245.248.128.0/20

The ISP wants to give half of this chunk of addresses to organization A, and a quarter to organization B, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to A and B?

(i) 245.248.136.0/21 and 245.248.128.0/22
(ii) 245.248.128.0/21 and 245.248.128.0/22
(iii) 245.248.132.0/22 and 245.248.132.0/21
(iv) 245.248.136.0/22 and 245.248.132.0/21

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Worked Solution
Answer

(i) 245.248.136.0/21 and 245.248.128.0/22

Step-by-step solution

Given block: 245.248.128.0/20.

A /20 IPv4 block contains:
$2^{32-20}=2^{12}=4096$ addresses.

So the complete range is:
245.248.128.0 to 245.248.143.255.

1. Organization A needs half

Half of 4096 = 2048 addresses.
A 2048-address block corresponds to /21 because:
$2^{32-21}=2048$ .

The valid /21 sub-blocks inside the /20 are:

245.248.128.0/21 → 245.248.128.0–245.248.135.255

245.248.136.0/21 → 245.248.136.0–245.248.143.255

2. Organization B needs one quarter

One quarter of 4096 = 1024 addresses.
A 1024-address block corresponds to /22 because:
$2^{32-22}=1024$ .

If A receives 245.248.136.0/21, the remaining lower half is 245.248.128.0–245.248.135.255. B can receive 245.248.128.0/22, which covers 245.248.128.0–245.248.131.255.

The ISP then retains 245.248.132.0–245.248.135.255, which is another /22 block.

Thus the allocations do not overlap and exactly satisfy the required 1/2 and 1/4 sizes.

Checking the options

(i) A = 245.248.136.0/21, B = 245.248.128.0/22 → valid, non-overlapping. ✅

(ii) A = 245.248.128.0/21 and B = 245.248.128.0/22 → overlap. ❌

(iii) A and B use overlapping address space and the block sizes/positions are invalid for the required allocation. ❌

(iv) A = 245.248.136.0/22 is only 1024 addresses, not the required 2048 for half. ❌

Final answer: (i) 245.248.136.0/21 and 245.248.128.0/22.
Q1b. Which one of the following tasks is not done by data link layer? (i) Framing (ii) Error control (iii) Flow control (iv) Channel coding20202m
Module 2: Data Link Layer & MAC Sublayer
Which one of the following tasks is not done by data link layer?

(i) Framing
(ii) Error control
(iii) Flow control
(iv) Channel coding

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Worked Solution
Answer

(iv) Channel coding

Explanation

The Data Link layer is responsible for functions such as framing, error control, and flow control. Channel coding, in the sense of encoding bits for reliable physical transmission over a medium, is primarily associated with the Physical layer.

Framing: Data Link layer groups the bit stream into frames.

Error control: Data Link layer can detect/correct errors or arrange retransmission.

Flow control: Data Link protocols can prevent a fast sender from overwhelming a receiver on a link.

Channel coding: Primarily a Physical-layer transmission function.

Final answer: (iv) Channel coding.
Q1c. Frames from one LAN can be transmitted to another LAN via the device (i) Router (ii) Bridge (iii) Repeater (iv) Modem20202m
Module 1: Physical Layer & Data Communication
Frames from one LAN can be transmitted to another LAN via the device

(i) Router
(ii) Bridge
(iii) Repeater
(iv) Modem

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Worked Solution
Answer

(ii) Bridge

Explanation

A bridge operates at the Data Link layer and connects LAN segments. It forwards Ethernet frames between LANs using MAC addresses.

Bridge: connects LAN segments and forwards frames based on MAC addresses.

Router: operates at the Network layer and forwards packets between IP networks.

Repeater: regenerates physical-layer signals but does not make forwarding decisions based on addresses.

Modem: performs modulation/demodulation for transmission over certain communication media.

Because the question specifically asks about transmitting frames from one LAN to another LAN, the expected answer is Bridge.

Final answer: (ii) Bridge.
Q1d. ______ refers to the physical or logical arrangement of a network. (i) Topology (ii) Mode of operation (iii) Data flow (iv) None of the above20202m
Module 1: Physical Layer & Data Communication
______ refers to the physical or logical arrangement of a network.

(i) Topology
(ii) Mode of operation
(iii) Data flow
(iv) None of the above

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Worked Solution
Answer

(i) Topology

Explanation

Network topology describes the physical or logical arrangement of devices and communication links in a network.

Examples include:

Bus: devices share a common backbone.

Star: devices connect to a central switch/hub.

Ring: devices form a logical/physical ring.

Mesh: devices have multiple interconnections.

Therefore, the correct answer is (i) Topology.
Q1e. In pure ALOHA, the vulnerable time is ______ the frame transmission time. (i) the same as (ii) two times (iii) three times (iv) None of the above20202m
Module 2: Data Link Layer & MAC Sublayer
In pure ALOHA, the vulnerable time is ______ the frame transmission time.

(i) the same as
(ii) two times
(iii) three times
(iv) None of the above

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Worked Solution
Answer

(ii) Two times

Explanation

In pure ALOHA, a frame can be transmitted at any time. Suppose a frame takes time $T$ to transmit.

For another frame to avoid collision with it, another transmission must not begin during the interval from $T$ before the frame starts until $T$ after the frame starts.

Therefore the vulnerable period is:

$T_v = 2T$

So the vulnerable time is twice the frame transmission time.

This is also why the maximum theoretical throughput of pure ALOHA is approximately $18.4\%$ .

Final answer: (ii) Two times.
Q1f. What is the default subnet mask for a class A network? (i) 127.0.0.1 (ii) 255.0.0.0 (iii) 255.255.255.0 (iv) 255.255.255.020202m
Module 3: Network Layer & Routing
What is the default subnet mask for a class A network?

(i) 127.0.0.1
(ii) 255.0.0.0
(iii) 255.255.255.0
(iv) 255.255.255.0

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Worked Solution
Answer

(ii) 255.0.0.0

Explanation

In the traditional classful IPv4 addressing scheme, a Class A network uses:

8 bits for the network portion.

24 bits for the host portion.

Therefore its default prefix is /8.

The corresponding subnet mask is:

$255.0.0.0$

In binary:

11111111.00000000.00000000.00000000

Thus the correct answer is (ii) 255.0.0.0.

Note: 127.0.0.0/8 is reserved for loopback, and 255.255.255.0 is a /24 mask, not the default Class A mask.
Q1g. What is the port number for POP3? (i) 110 (ii) 90 (iii) 80 (iv) 4920202m
Module 5: Application Layer & Security
What is the port number for POP3?

(i) 110
(ii) 90
(iii) 80
(iv) 49

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Q1h. The IEEE 802 project of the 1980s involved further defining the lower two layers of the OSI model. A number of standards were agreed upon during that time. Which of the following is the standard for ethernet? (i) 802.2 (ii) 802.3 (iii) 802.4 (iv) 802.520202m
Module 1: Physical Layer & Data Communication
The IEEE 802 project of the 1980s involved further defining the lower two layers of the OSI model. A number of standards were agreed upon during that time. Which of the following is the standard for ethernet?

(i) 802.2
(ii) 802.3
(iii) 802.4
(iv) 802.5

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Q1i. The first network is (i) CNNET (ii) NSFNET (iii) ASAPNET (iv) ARPANET20202m
Module 1: Physical Layer & Data Communication
The first network is

(i) CNNET
(ii) NSFNET
(iii) ASAPNET
(iv) ARPANET

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Q1j. TCP (i) operates at the data link layer (ii) is connection-oriented and unreliable (iii) is connection-oriented and reliable (iv) is connectionless and unreliable20202m
Module 4: Transport Layer & Congestion Control
TCP

(i) operates at the data link layer
(ii) is connection-oriented and unreliable
(iii) is connection-oriented and reliable
(iv) is connectionless and unreliable

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Q2a. What is the importance of physical layer? Discuss the main duties of the physical layer.20207m
Module 1: Physical Layer & Data Communication
What is the importance of physical layer? Discuss the main duties of the physical layer.

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Q2b. Explain how sliding window protocol works at data link layer.20207m
Module 2: Data Link Layer & MAC Sublayer
Explain how sliding window protocol works at data link layer.

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Q3a. Explain random access protocol. Differentiate between pure ALOHA and slotted ALOHA.20207m
Module 2: Data Link Layer & MAC Sublayer
Explain random access protocol. Differentiate between pure ALOHA and slotted ALOHA.

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Q3b. Explain the different types of Ethernet.20207m
Module 1: Physical Layer & Data Communication
Explain the different types of Ethernet.

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Q4a. How do guided media differ from unguided media? Name the advantages of optical fiber over twisted-pair and coaxial cable.20207m
Module 1: Physical Layer & Data Communication
How do guided media differ from unguided media? Name the advantages of optical fiber over twisted-pair and coaxial cable.

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Q4b. Why does the transport layer take measures to control congestion when it is primarily handled at the network layer? TCP is provided with techniques to control congestion, but not UDP. Why?20207m
Module 4: Transport Layer & Congestion Control
Why does the transport layer take measures to control congestion when it is primarily handled at the network layer? TCP is provided with techniques to control congestion, but not UDP. Why?

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Q5a. Describe about services provided by the data-link layer.20207m
Module 2: Data Link Layer & MAC Sublayer
Describe about services provided by the data-link layer.

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Q5b. What are the advantages of dividing an Ethernet LAN with a bridge? Why is there no need for CSMA/CD on a full-duplex Ethernet LAN? The address 43:78:6C:DE:10:00 has been shown as the source address in an Ethernet frame. The receiver has discarded the frame. Why?20207m
Module 2: Data Link Layer & MAC Sublayer
What are the advantages of dividing an Ethernet LAN with a bridge? Why is there no need for CSMA/CD on a full-duplex Ethernet LAN? The address 43:78:6C:DE:10:00 has been shown as the source address in an Ethernet frame. The receiver has discarded the frame. Why?

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Q6b. An IP datagram is carrying a TCP segment destined for address 130.14.16.17/16. The destination port address is corrupted, and it arrives at destination 130.14.16.19/16. How does the receiving TCP react to this error? Show the entries for the header of a TCP segment that carries a message from an FTP client to an FTP server. Fill the checksum field with Os. Choose an appropriate ephemeral port number and the correct well-known port number. The length of the data is 40 bytes.20207m
Module 4: Transport Layer & Congestion Control
An IP datagram is carrying a TCP segment destined for address 130.14.16.17/16. The destination port address is corrupted, and it arrives at destination 130.14.16.19/16. How does the receiving TCP react to this error? Show the entries for the header of a TCP segment that carries a message from an FTP client to an FTP server. Fill the checksum field with Os. Choose an appropriate ephemeral port number and the correct well-known port number. The length of the data is 40 bytes.

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Q8a. A TCP connection is using a window size of 10,000 bytes, and the previous acknowledgment number was 22,001. It receives a segment with acknowledgment number 24,001 and window size advertisement of 12,000. Draw a diagram to show the situation of the window before and after.20207m
Module 4: Transport Layer & Congestion Control
A TCP connection is using a window size of 10,000 bytes, and the previous acknowledgment number was 22,001. It receives a segment with acknowledgment number 24,001 and window size advertisement of 12,000. Draw a diagram to show the situation of the window before and after.

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Q8b. Show the entries for the header of a TCP segment that carries a message from an FTP client to an FTP server. Fill the checksum field with Os. Choose an appropriate ephemeral port number and the correct well-known port number. The length of the data is 40 bytes.20207m
Module 4: Transport Layer & Congestion Control
Show the entries for the header of a TCP segment that carries a message from an FTP client to an FTP server. Fill the checksum field with Os. Choose an appropriate ephemeral port number and the correct well-known port number. The length of the data is 40 bytes.

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Q9a. What kinds of file types can FTP transfer? What are the three FTP transmission modes? Describe the functions of the two FTP connections. How does storing a file differ from retrieving a file?20207m
Module 5: Application Layer & Security
What kinds of file types can FTP transfer? What are the three FTP transmission modes? Describe the functions of the two FTP connections. How does storing a file differ from retrieving a file?

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Q9b. **An ISP is granted the block 80.70.56.0/21. The ISP needs to allocate addresses for two organizations each with 500 addresses, two organizations each with 250 addresses, and three organizations each with 50 addresses. (i) Find the number and range of addresses in the ISP block. (ii) Find the range of addresses for each organization and the range of unallocated addresses.**20207m
Module 3: Network Layer & Routing
**An ISP is granted the block 80.70.56.0/21. The ISP needs to allocate addresses for two organizations each with 500 addresses, two organizations each with 250 addresses, and three organizations each with 50 addresses.

(i) Find the number and range of addresses in the ISP block.
(ii) Find the range of addresses for each organization and the range of unallocated addresses.**

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Computer Networks

Answer

Step-by-step solution

1. Organization A needs half

2. Organization B needs one quarter

Checking the options

Answer

Explanation

Answer

Explanation

Answer

Explanation

Answer

Explanation

Answer

Explanation