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Birla Institute of Technology & Science, Pilani

Distance Learning Programmes Division

Second Semester 2006-2007

Comprehensive Examination (EC-2 Regular)

Course No. : BITS ZC481

Course Title : COMPUTER NETWORKS

Nature of Exam : Open Book 

Weightage : 60%

Duration : 3 Hours

Date of Exam : 31/03/2007 (FN)Note:1.  Please follow all the Instructions to Candidates given on the cover page of the answer book.

2.  All parts of a question should be answered consecutively. Each answer should start from a fresh page.

Q.1  For the network given below, give the distance-vector routing table at node B when

(a)  Each node knows only the distances to its immediate neighbors.

(b)  Each node has reported the information it had in the preceding step to its

immediate neighbors.

(c)  Step (b) happens a second time.

(d)  After the routing tables had fully converged, node B fails. Give the distance of 

node B from all other nodes after three exchanges of routing tables have taken

place. Show the distance details after each exchange. [3 + 3 + 3 + 6 = 15]

Q.2  A TCP connection is made between two nodes on a network. Assume a 100msec RTT and

a 120sec maximum segment lifetime.

(a) Identify the two problems that occur as link bandwidth increases.

(b) At what bandwidths do each of these two problems become an issue? [5 + 5 = 10]

Q.3  Answer the following briefly (pointwise):

(a)  Describe four different scenarios when an IP router would drop packets that arrive

on one of its interfaces.

(b)  List four fields of a packet that an IP router must modify before forwarding the

packet to its next hop. [5 + 5 = 10]

Q.4 (a)  The Diffie-Hellman key exchange is being used to establish a secret key between

Alice and Bob. Alice sends Bob (719,3,191). Bob responds with (543). Alice’s secret

number, x, is 16. What is the secret key? [5]

Q.4 (b)  A binary file is 3072 bytes long. How long will it be if encoded using base64

encoding, with a CR+LF pair inserted after every 80 bytes sent and at the end? [5]

Q5. Assume that you have been assigned the 200.35.1.0 /24 network block.

a)  Define an extended network prefix that allows the creation of 40 hosts on each

subnet.

b)  What is the maximum number of hosts that can be assigned to each subnet?

c)  What is the maximum number of subnets that can be defined?

d)  Specify the subnets of 200.35.1.0/24 in dotted decimal notation.

e)  List the range of host addresses that can be assigned to each of the first four subnets.

f)  What is the broadcast address for each of the first four subnet

[2 + 1 + 1 + 4 + 4 + 3 = 15] 

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No. of Pages = 1

No. of Questions = 5