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Authors:
James F. Kurose, Keith W. Ross
Chapter:
The Network Layer
Exercise:
Problems
Question:17 | ISBN:9780132856201 | Edition: 6

Question

Consider the topology shown in Figure 4.17. Denote the three subnets with hosts (starting clockwise at 12:00) as Networks A, B, and C. Denote the sub- nets without hosts as Networks D, E, and F.

a. Assign network addresses to each of these six subnets, with the follow- ing constraints: All addresses must be allocated from 214.97.254/23; Subnet A should have enough addresses to support 250 interfaces; Sub- net B should have enough addresses to support 120 interfaces; and Subnet C should have enough addresses to support 120 interfaces. Of course, subnets D, E and F should each be able to support two interfaces. For each subnet, the assignment should take the form a.b.c.d/x or a.b.c.d/x – e.f.g.h/y.

b. Using your answer to part (a), provide the forwarding tables (using longest prefix matching) for each of the three routers.

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Answer


a)

Subnet A: 214.97.255/24 (256 addresses)

Subnet B: 214.97.254.0/25 - 214.97.254.0/29 (128-8 = 120 addresses)

Subnet C: 214.97.254.128/25 (128 addresses)


Subnet D: 214.97.254.0/31 (2 addresses)

Subnet E: 214.97.254.2/31 (2 addresses)

Subnet F: 214.97.254.4/30 (4 addresses)


b)

To simplify the solution, assume that no data grams have router interfaces as ultimate destinations. Also, label D, E, F for the upper-right, bottom, and upper-left interior subnets, respectively.

Router 1:

Longest Prefix Match Outgoing Interface

----------------------------------------------------------------------------------------

11010110 01100001 11111111 Subnet A

11010110 01100001 11111110 0000000 Subnet D

11010110 01100001 11111110 000001 Subnet F

Router 2:

Longest Prefix Match Outgoing Interface

----------------------------------------------------------------------------------------

11010110 01100001 11111111 0000000 Subnet D

11010110 01100001 11111110 0 Subnet B

11010110 01100001 11111110 0000001 Subnet E




Router 3:

Longest Prefix Match Outgoing Interface

----------------------------------------------------------------------------------------

11010110 01100001 11111111 000001 Subnet F

11010110 01100001 11111110 0000001 Subnet E

11010110 01100001 11111110 1 Subnet C



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