50 Computer Networks MCQs with Answers

Question 21

Consider a source computer(S) transmitting a file of size 10⁶ bits to a destination computer(D)over a network of two routers (R₁ and R₂) and three links(L₁, L₂, and L₃). L₁connects S to R₁; L₂ connects R₁ to R₂; and L₃ connects R₂ to D.Let each link be of length 100 km. Assume signals travel over each link at a speed of 10⁸ meters per second.Assume that the link bandwidth on each link is 1Mbps. Let the file be broken down into 1000 packets each of size 1000 bits. Find the total sum of transmission and propagation delays in transmitting the file from S to D?

• 1005 ms
• 1010 ms
• 3000 ms
• 3003 ms

Question 22

Consider a network with five nodes, N1 to N5, as shown below. The network uses a Distance Vector Routing protocol. Once the routes have stabilized, the distance vectors at different nodes are as following. N1: (0, 1, 7, 8, 4) N2: (1, 0, 6, 7, 3) N3: (7, 6, 0, 2, 6) N4: (8, 7, 2, 0, 4) N5: (4, 3, 6, 4, 0) Each distance vector is the distance of the best known path at the instance to nodes, N1 to N5, where the distance to itself is 0. Also, all links are symmetric and the cost is identical in both directions. In each round, all nodes exchange their distance vectors with their respective neighbors. Then all nodes update their distance vectors. In between two rounds, any change in cost of a link will cause the two incident nodes to change only that entry in their distance vectors. 52. The cost of link N2-N3 reduces to 2(in both directions). After the next round of updates, what will be the new distance vector at node, N3.

• (3, 2, 0, 2, 5)
• (3, 2, 0, 2, 6)
• (7, 2, 0, 2, 5)
• (7, 2, 0, 2, 6)

Question 23

Consider the data given in above question. Suppose the weights of all unused links in the previous question are changed to 2 and the distance vector algorithm is used again until all routing tables stabilize. How many links will now remain unused?

• 0
• 1
• 2
• 3

Question 24

Classless Inter-domain Routing (CIDR) receives a packet with address 131.23.151.76. The router’s routing table has the following entries:

Prefix           Output Interface Identifier
131.16.0.0/12              3
131.28.0.0/14              5
131.19.0.0/16              2
131.22.0.0/15              1

The identifier of the output interface on which this packet will be forwarded is ______.

• 1
• 2
• 3
• 5

Question 25

Two computers C1 and C2 are configured as follows. C1 has IP address 203.197.2.53 and netmask 255.255.128.0. C2 has IP address 203.197.75.201 and netmask 255.255.192.0. which one of the following statements is true?

• C1 and C2 both assume they are on the same network
• C2 assumes C1 is on same network, but C1 assumes C2 is on a different network
• C1 assumes C2 is on same network, but C2 assumes C1 is on a different network
• C1 and C2 both assume they are on different networks.

Question 26

A Trojan horse is

• A program that performs a legitimate function that is known to an operating system or its user and also has a hidden component that can be used for nefarious purposes like attacks on message security or impersonation.
• A piece of code that can attach itself to other programs in the system and spread to other systems when programs are copied or transferred.
• A program that spreads to other computer systems by exploiting security holes like weaknesses in facilities for creation of remote processes
• All of the above

Question 27

In cryptography, the following uses transposition ciphers and the keyword is LAYER. Encrypt the following message. (Spaces are omitted during encryption) WELCOME TO NETWORK SECURITY !

• WMEKREETSILTWETCOOCYONRU!
• EETSICOOCYWMEKRONRU!LTWET
• LTWETONRU!WMEKRCOOCYEETSI
• ONRU!COOCYLTWETEETSIWMEKR

Question 28

The minimum positive integer p such that 3^p modulo 17 = 1 is

• 5
• 8
• 12
• 16

Question 29

SHA-1 is a

• encryption algorithm
• decryption algorithm
• key exchange algorithm
• message digest function

Question 30

If there are N people in the world and are using secret key encryption/decryption for privacy purpose, then number of secret keys required will be:

• N
• (N - 1)
• N (N - 1) / 2
• N (N + 1) / 2