PAPER CODE NO. EXAMINER: Martin Gairing Tel. No.
0151 795 4264
COMP211 DEPARTMENT: Computer Science
MOCK EXAMINATION
Computer Networks
TIME ALLOWED : TWO Hours
INSTRUCTIONS TO CANDIDATES
NAME OF CANDIDATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEAT NO . . . . . . . . . . . . . . . .
USUAL SIGNATURE ...........................................
READ THE FOLLOWING CAREFULLY:
1. Each of the following questions comprise 5 statements, for which you should select the one
most appropriate answer by placing a tick in the appropriate box.
2. The exam mark is based on the overall number of correctly answered questions. The more
questions you answer correctly the higher your mark, incorrectly answered questions do not
count against you.
3. Enter your name and examination number IN PENCIL on the computer answer sheet
according to the instructions on that sheet.
4. When you have completed this exam paper, read the instructions on the computer answer
sheet carefully and transfer your answers from the exam paper. Use a HB pencil to mark the
computer answer sheet and if you change your mind be sure to erase the mark you have
made. You may then mark the alternative answer.
5. At the end of the examination, be absolutely sure to hand in BOTH this exam paper AND the
computer answer sheet.
6. Calculators are NOT permitted.
THIS PAPER MUST NOT BE REMOVED FROM THE EXAMINATION ROOM
PAPER CODE COMP211 page 1 of 6 Continued
1. The layer has the primary responsibility for communications between one device
and the devices connected to it.
A. network
B. physical
C. transport
D. link
E. application
2. A network router joins two together?
A. computers
B. networks
C. switches
D. gateways
E. hosts
3. What is the size of an IPv4 address?
A. 32 bit
B. 48 bit
C. 64 bit
D. 80 bit
E. 128 bit
4. As a message proceeds down the protocol stack, the destination port number is added
at the:
A. Application Layer
B. Transport Layer
C. Network Layer
D. Data Link Layer
E. None of the above
5. What is the port number for SMTP?
A. 21
B. 25
C. 65
D. 70
E. 80
PAPER CODE COMP211 page 2 of 6 Continued
6. In the DNS structure, the servers responsible for the .uk domain are name
servers.
A. authoritative
B. top level domain
C. root
D. default
E. none of the above
7. Which of the following applications will effectively run on UDP?
A. Email
B. File transfer
C. Web
D. Multimedia streaming
E. All of the above
8. In a client/server connection using HTTP over TCP, if multiple objects are sent over the
same TCP connection, then the connection is classified as:
A. Stateless
B. Persistant
C. Non-persistant
D. Stable
E. Reliable
9. Consider the Go-Back-N protocol with a sender window size of N = 4 and a sequence
number range of 1024. Suppose that at time t the next in-order packet that the receiver
is expecting has sequence number 228. Assume that the medium does not reorder
messages. Which of the following is a possible pair of values for the senders send base
and nextseqnum pointers at time t?
A. send base=224; nextseqnum=226
B. send base=225; nextseqnum=228
C. send base=227; nextseqnum=232
D. send base=228; nextseqnum=233
E. send base=229; nextseqnum=233
PAPER CODE COMP211 page 3 of 6 Continued
10. Packet retransmission in TCP is coupled with:
A. Flow Control
B. Congestion Control
C. Connection Management
D. All of the above
E. None of the above
11. TCP and UDP use the 16-bit Internet Checksum for computing the value of the checksum
field in each segment. In this question you are asked to use the same method but for
simplicity we restrict to 8-bit words. Suppose you have the following three 8-bit words:
11101011
01010101
10110111
What is the value of the 8-bit Internet Checksum (i.e., the 1s complement of the 8-bit
sum) of these words?
A. 00000111
B. 00001000
C. 00001010
D. 00001001
E. None of the above
12. Consider a router that interconnects three subnets: X, Y, and Z. Suppose all of the in-
terfaces in each of these subnets are required to have the prefix 13.2.80.0/21. Suppose
subnet X is required to support 1000 interfaces, and subnets Y and Z are each required
to support 500 interfaces. Which set of network addresses for X,Y and Z satisfies these
constraints?
A. X: 13.2.80.0/22; Y: 13.2.82.0/23; Z: 13.2.84.0/23
B. X: 13.2.80.0/22; Y: 13.2.84.0/23; Z: 13.2.86.0/23
C. X: 13.2.84.0/23; Y: 13.2.80.0/23; Z: 13.2.82.0/23
D. X: 13.2.88.0/23; Y: 13.2.84.0/22; Z: 13.2.80.0/22
E. None of the above
PAPER CODE COMP211 page 4 of 6 Continued
13. Given are the following subnet addresses:
X: 192.168.128.0/22 Y: 192.168.136.0/22 Z: 192.168.142.0/22
Which of the following statements is true?
A. 192.168.132.1 and 192.168.146.1 are not in the IP range of any of the 3 subnets.
B. 192.168.132.1 is in the range of X, and
192.168.146.1 is in the range of Z.
C. 192.168.132.1 is in the range of Y, and
192.168.146.1 is in the range of Z.
D. 192.168.132.1 is is not in the IP range of any of the 3 subnets, and
192.168.146.1 is in the range of Z.
E. 192.168.132.1 is in the range of X, and
192.168.146.1 is not in the IP range of any of the 3 subnets.
14. When the speed of the incoming packets at a router exceeds the outgoing link data rate,
which of the following may occur?
A. Transmission delay
B. Queuing delay
C. Packet loss
D. All of the above
E. None of the above
15. Which of the following MAC protocols is used by Ethernet?
A. Slotted Aloha
B. CSMA/CA
C. CSMA/CD
D. TDMA
E. FDMA
16. What are the CRC bits defined by the generator 1101 and the data bit string 111011?
A. 011
B. 100
C. 101
D. 110
E. 111
PAPER CODE COMP211 page 5 of 6 Continued
17. What is the signal-to-noise ratio corresponding to 40dB?
A. 40
B. 400
C. 1000
D. 4000
E. 10000
18. What is the main advantage of first distributing a session key and then using symmetric-
key cryptography rather than using public-key cryptography techniques for the whole
communication?
A. Symmetric-key cryptography is more secure
B. Public-key cryptography is more secure
C. Public-key cryptography leads to shorter cyphertext
D. Symmetric-key cryptography is faster than public-key cryptography
E. None of the above
19. Consider RSA with p = 5 and q = 7. What are the values of n and z?
A. n = 35 and z = 24
B. n = 35 and z = 96
C. n = 119 and z = 24
D. n = 119 and z = 96
E. None of the above
20. The AES encryption standard uses the following techniques:
A. Block Cypher
B. Stream Cypher
C. Public key cryptography
D. A and B
E. All of the above
PAPER CODE COMP211 page 6 of 6 End
