Computer Networks test questions for interview, exams, entrance

Computer Networks test questions for interview, exams, entrance


(1) We have two hosts A and B that are using sliding window protocol for flow control, and to transfer data from A to B full duplex link is used. The send window size and receive window size is 5 packets each. Data packets that are sent only from A to B are all 1000byte long and the transmission time for such a packet is 50µs. Acknowledgement packets that are sent only from B to A are very small and it requires negligible transmission time. In this communication the maximum achievable throughput, if the propagation delay over the link is 200µs, is

(A) 9.12 x 106 bps
(B) 10.09 x 106 bps
(C) 12.33 x 106 bps
(D) 11.11 x 106 bps

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ANSWER: 11.11 x 106 bps




(2) The different activities pertaining to email are:

a. Send an email from a mail client to mail server.
b. Download an email from mailbox server to a mail client.
c. Checking email in a web browser.

In each activity given above, the application level protocol used, is

(A) a: SMTP, b: POP, c: HTTP
(B) a: SMTP, b: HTTP, c: POP
(C) a: POP, b: SMTP, c: HTTP
(D) a: SMTP, b: IMAP, c: POP

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ANSWER: a: SMTP, b: POP, c: HTTP




(3) Which one of the following statement does not hold true about Internet protocol (IP)?

(A) IP packets from the same source to the same destination can take different routes in the network
(B) If the packet is unable to reach its destinations within a given number of hops, IP ensures that a packet is discarded
(C) A computer can have multiple IP address
(D) All of the above

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ANSWER: A computer can have multiple IP address




(4) Let computer A and computer B that have the IP addresses 10.105.1.113 and 10.105.1.91 respectively use the same net mask N. For A and B to belong to same network, the value of N should not be

(A) 255.255.255.224
(B) 255.255.255.0
(C) 255.255.255.128
(D) 255.255.255.192

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ANSWER: 255.255.255.224




For questions 5 and 6 refer to the data given below:

Let A, B and C be the three IP networks and Ha, Hb and Hc be the hosts of network A, network B and network c respectively. Message containing 180byte of application data is send by host Ha to host Hc through an intermediate network B. The TCP layer prefixes a 20byte header to the message. In each network, the maximum packet size including 20byte IP header is A: 1000byte, B: 100byte, C: 1000byte. The network A is connected to network B through a 1Mbps link and network B is connected to network C through a 512kbps link, where bps stands for bits per second.

(5) Consider the packets are correctly delivered. In the best case, how many bytes including headers are delivered to the IP layer at the destination, for one application message? Consider only data packets.

(A) 200
(B) 220
(C) 240
(D) 260

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ANSWER: 260




(6) Neglecting errors, acknowledgements and other overheads, the rate at which application data is transferred to host Hc, is

(A) 150.8kbps
(B) 245.6kbps
(C) 400.9kbps
(D) 354.5kbps

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ANSWER: 354.5kbps




(7) For real time multimedia, file transfer, DNS and email, the transport layer protocols used are respectively

(A) TCP, UDP, TCP and UDP
(B) UDP, UDP, TCP and TCP
(C) UDP, TCP, UDP and TCP
(D) TCP, TCP, UDP and UDP

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ANSWER: UDP, TCP, UDP and TCP




(8) Match the following:



(A) P – 1, Q – 4, R – 2, S - 3
(B) P – 4, Q – 3, R – 2, S - 1
(C) P – 2, Q – 1, R – 4, S - 3
(D) P – 4, Q – 3, R – 1, S - 2

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ANSWER: P – 1, Q – 4, R – 2, S - 3




(9) A device that can look at all protocol headers up to the transport layer is called l ayer-4 firewall. Which one of the following statement is true layer-4 firewall?

(A) It cannot block all ICMP traffic
(B) It cannot block entire HTTP traffic during 9:00pm and 5:00am
(C) It cannot block TCP traffic from a specific user on a multi-user system during 9:00pm and 5:00am
(D) It cannot stop incoming traffic from a specific IP address but allow outgoing traffic to the same address

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ANSWER: It cannot block TCP traffic from a specific user on a multi-user system during 9:00pm and 5:00am




(10) The source and destination are connected through two intermediate routers. During the data transmission from source to destination how many times each packet has to visit the network layer and the data link layer?

(A) Network layer – 6 times and data link layer – 4 times
(B) Network layer – 4 times and data link layer – 2 times
(C) Network layer – 4 times and data link layer – 6 times
(D) Network layer – 2 times and data link layer – 3 times

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ANSWER: Network layer – 4 times and data link layer – 6 times




(11) For the application layer in the Internet stack, the protocol data unit (PDU) is

(A) Datagram
(B) Message
(C) Frame
(D) Segment

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ANSWER: Message




(12) In the Ipv4 addressing format, how many networks are allowed under class C addresses?

(A) 221
(B) 216
(C) 224
(D) 27

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ANSWER: 221




(13) The transport layer protocols that is used to support electronic mail is

(A) TCP
(B) SMTP
(C) UDP
(D) IP

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ANSWER: TCP




(14) We have an Ipv4 datagram with following specifications:

M bit = 0,
The value is the of HLEN = 10,
The value of total length = 400,
The fragment offset value = 300,

What is the position of the datagram, the sequence numbers of the first and last bytes of the payload respectively?

(A) First fragment, 2400 and 2759
(B) Middle fragment, 300 and 689
(C) Last fragment, 2400 and 2789
(D) Last fragment, 2400 and 2759

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ANSWER: Last fragment, 2400 and 2759




(15) Consider a broadcast LAN whose length is 2km, bandwidth is 107 bps and it uses CSMA/CD. If the traveling speed of the signal along the wire is 2 x 108 m/s then the minimum packet size that could be used on this network is

(A) 50bytes
(B) 100bytes
(C) 200bytes
(D) None of the above

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ANSWER: None of the above




(16) The data is to be transmitted at the rate of 500Mbps in an Ethernet LAN with frame size of 10,000bits. Assuming the signal speed in the cable is 2,00,000km/s the maximum length of the cable for transmitting data is

(A) 1km
(B) 2km
(C) 5km
(D) 10km

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ANSWER: 2km




(17) The pair of IP addresses that could belong to a network whose subnet mask is 255.255.31.0 is

(A) 10.35.28.2 and 10.35.29.4
(B) 172.57.88.62 and 172.56.87.233
(C) 128.8.129.43 and 128.8.161.55
(D) 191.203.31.87 and 191.243.31.88

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ANSWER: 128.8.129.43 and 128.8.161.55




(18) Ethernet = 10Mbps, Jamming signal = 48bit, Round trip propagation delay = 46.4μs, minimum frame size =?

(A) 512
(B) 440
(C) 100
(D) 350

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ANSWER: 512




(19) The table given below is the routing table of a router:



The interface where the router forward packets addressed to destinations 1 28.75.43.16 and 192.12.17.10 respectively is

(A) Eth0 and Eth2
(B) Eth0 and Eth3
(C) Eth1 and Eth3
(D) Eth1 and Eth2

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ANSWER: Eth0 and Eth3




(20) A message that consists of 9 packets is to be send by station A to station B, using sliding window (window size 3) and go-back-n error control strategy. All packets are ready and immediately available for transmission. The number of packets A will transmit for sending the message to B, if every 5th packet that is transmitted by A gets lost (but no acts from B ever

gets lost), is


(A) 20
(B) 9
(C) 15
(D) 12

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ANSWER: 12




(21) On an Ethernet only two stations, station A and station B are present and each station has a steady queue of frames to send. In an attempt to transmit a frame both A and B collide and A wins the first backoff race. At the end of this successful transmission by A, both A and B attempt to transmit and collide. What is the probability that A wins the second

backoff race is


(A) 0.625
(B) 0.5
(C) 1.0
(D) 0.75

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ANSWER: 0.625




(22) Consider a packet switching network where packets are routed from source to destination along a simple path having two intermediate nodes. What will be the optimum packet size if the message size is 24 byte and each packet contains a header of 3 byte?

(A) 8
(B) 7
(C) 9
(D) 5

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ANSWER: 9




(23) Let A and B be the two computers that are configured as follows:

IP address of A = 203.197.2.53
Netmask of A = 255.255.128.0
IP address of B = 203.197.75.201
Netmask of B = 255.255.192.0

The statement that holds true is

(A) B assumes A on the same network but A assumes B on different network
(B) A assumes B on the same network but B assumes A on different network
(C) Both A and B assumes they are on the same network
(D) Both A and B assumes they are on the different network

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ANSWER: A assumes B on the same network but B assumes A on different network




(24) To protect the message 11001001 from errors while transmitting, the CRC polynomial x³ + 1 is used. What message should be transmitted?

(A) 11001001011
(B) 11001010
(C) 11001001000
(D) 110010010011

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ANSWER: 11001001011




(25) To transmit messages to station B, station A uses 32 byte packets and a sliding window protocol. The round trip delay between A and B is 80ms and the bottleneck bandwidth on the path between A and B is 128 kbps. The optimal size of the window, A should have is

(A) 440
(B) 100
(C) 320
(D) 40

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ANSWER: 40




(26) The address of a class B host is to be split into subnets with a 6-bit subnet number. The maximum number of subnets and the maximum number of hosts in each subnet is

(A) 62 subnets and 440 hosts
(B) 62 subnets and 262142 hosts
(C) 62 subnets and 267812 hosts
(D) 62 subnets and 1022 hosts

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ANSWER: 62 subnets and 1022 hosts




(27) We have a slotted LAN with n number of stations. P is the probability each station attempts to transmit in each time slot. The probability that only one station transmits in a given time slot is

(A) 1 - (1-p) n-1
(B) (1-p) n-1
(C) np (1-p) n-1
(D) p (1-p) n-1

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ANSWER: np (1-p) n-1




(28) Consider a class B network on the Internet that has a subnet mask of 255.255.248.0. The maximum number of hosts per subnet is

(A) 2047
(B) 2046
(C) 1023
(D) 1024

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ANSWER: 2046




(29) The token bucket, which is filled at a rate of 2Mbps, regulates a computer on a 10Mbps network. It is initially filled to capacity with 16Mbits. The maximum duration for which the computer can transmit at the full 10Mbps is

(A) 5s
(B) 4s
(C) 3s
(D) 2s

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ANSWER: 2s




For questions 30 and 31 refer to the data given below:

Consider a 106 bps duplex link between two hosts. Frames of 1000 bits are to be transmitted into this link to maximally pack them in transit (within the link). The propagation time

is 25 ms.

(30) Assuming no time gap between transmission of two frames, the minimum number of bits required to represent the sequence numbers distinctly is

(A) 5
(B) 4
(C) 3
(D) 2

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ANSWER: 5




(31) The size of the sender window is 21 where 1 is the number of bits identified in the earlier part and acknowledgements are always piggy backed. The sliding window protocol is used with this sender window. After sending 21 frames, the minimum time the sender will have to wait before starting transmission of the next frame is (Identify the closest

choice ignoring the frame processing time


(A) 14ms
(B) 16ms
(C) 18ms
(D) 20ms

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ANSWER: 18ms




(32) The transmission speed and the propagation speed in a token ring network is 107 bps and 200m/µs respectively. The 1-bit delay in this network is equivalent to

(A) 5m of cable
(B) 20 m of cable
(C) 100 m of cable
(D) 500 m of cable

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ANSWER: 20 m of cable




(33) The size of the congestion window in the slow start phase of the TCP congestion algorithm

(A) Decreases exponentially
(B) Increases exponentially
(C) Increases linearly
(D) Does not increase

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ANSWER: Increases exponentially




(34) For CRC checking generator polynomial G (x) is used. In order to detect odd number of bits in error, G (x) should satisfy the condition

(A) (1 + x) is a factor of G (x)
(B) G (x) has an odd number of terms
(C) G (x) contains more than two terms
(D) It is not necessary to satisfy any condition

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ANSWER: (1 + x) is a factor of G (x)




(35) Consider a network of LAN’s, which is connected by bridges. Intermediate bridges are used to transfer the packets from one LAN to another. Between two LAN’s as more than one path may exists packets may have to be routed through multiple bridges. Which one of the following reason is correct in order to use spanning tree algorithm for bridge routing?

(A) For fault tolerance
(B) For shortest path routing between LAN’s
(C) To minimize collisions
(D) To avoid loops in the routing paths

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ANSWER: To avoid loops in the routing paths




(36) A 9600 baud serial communication link uses asynchronous mode of transmission with one start bit, eight data bits, two stop bits and one parity bit. The number of 8-bit characters that can be transmitted per second over this link is

(A) 800
(B) 500
(C) 1000
(D) 440

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ANSWER: 800




(37) In serial transmission why every byte of data is padded with a 0 in the beginning and one or two 1’s at the end of byte?

(A) Because receiver recovers lost 0’s and 1’s from these padded bits
(B) Because padded bits are useful in parity computation
(C) To synchronize the receiver for byte reception
(D) None of the above

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ANSWER: To synchronize the receiver for byte reception




(38) Match the following:



(A) a – 3, b – 2, c - 1
(B) a – 1, b – 3, c - 2
(C) a – 2, b – 3, c - 1
(D) a – 1, b – 2, c - 3


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ANSWER: a – 1, b – 3, c - 2




(39) The functionality that must be implemented by a transport protocol over and above the network protocol is

(A) End to end connectivity
(B) Recovery from packet losses
(C) Detection of duplicate packets
(D) Packet delivery in the correct order

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ANSWER: End to end connectivity




(40) With respect to a transparent bridge and routers the statement that does not hold true is

(A) A bridge builds up its routing table by inspecting incoming packets
(B) A router can connect between LAN and WAN
(C) Bridge uses IP address and routers uses MAC address
(D) Both bridge and router selectively forward data packets

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ANSWER: Bridge uses IP address and routers uses MAC address




(41) If an organization that has class-B network, wishes to form subnets for 64 departments, what would be the subnet mask?

(A) 255.255.64.0
(B) 255.255.0.0
(C) 255.255.1.28.0
(D) 255.255.255.0

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ANSWER: 255.255.255.0




(42) In an IP datagram one of the header fields is time to live (TTL) field because

(A) It can be used to prevent packet looping
(B) It can be used to optimize throughput
(C) It can be used to reduce delays
(D) It can be used to prioritize packets

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ANSWER: It can be used to prevent packet looping




(43) In ___________ packets of the same session may be routed through different paths

(A) UDP but not TCP
(B) TCP and UDP
(C) TCP but not UDP
(D) Neither TCP nor UDP

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ANSWER: TCP and UDP




(44) The system calls that results in the sending of SYN packets is

(A) Listen
(B) Socket
(C) Connect
(D) Bind

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ANSWER: Connect




(45) Why does the Internet protocol use time to live (TTL) field in the IP datagram header?

(A) Limit the time for which a packet gets queued in intermediate routers
(B) Prevent packets from looping indefinitely
(C) Ensure packets reach destination within that time
(D) Discard packets that reach later than that time

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ANSWER: Discard packets that reach later than that time




(46) What is the use of address resolution protocol (ARP)?

(A) To find the MAC address corresponding to an IP address.
(B) To find an IP address corresponding to the MAC address
(C) To find the IP address from the DNS
(D) To find the IP address of the default gateway

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ANSWER: To find the MAC address corresponding to an IP address.




(47) If Manchester encoding is used in the Ethernet then the bit rate is

(A) Same as the baud rate
(B) Half of the baud rate
(C) Twice the baud rate
(D) None of the above

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ANSWER: Twice the baud rate




(48) The application layer can pass data on to the TCP layer below. The maximum size of this data is

(A) 1500 byte
(B) 216 byte size of TCP header
(C) 216 byte
(D) Any size

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ANSWER: 216 byte size of TCP header




(49) UDP is used as a transport protocol by

(A) SMTP
(B) HTTP
(C) DNS
(D) Telnet

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ANSWER: DNS




(50) The data is to be transmitted using the selective reject protocol with n bit frame sequence number. What is the maximum window size for this transmission?

(A) 2n-1
(B) 2n-2
(C) 2n
(D) 2n -1

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ANSWER: 2n-1



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