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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

FIGURE 3.9

VLSM design example

192.168.55.2/30

S0/1:

RouterA

RouterB

        • 192.168.55.57

          /27

        • 192.168.55.29

          /28

        • 192.168.55.1

          /30

        • 192.168.55.132

          /25

192.168.55.0/30 192.168.55.127/26

RouterC

S0/0

F0/0:

F0/0:

F0/0:

7 hosts

90 hosts

23 hosts

From the list of IP addresses on the left of the figure, which IP address will be placed in each router’s FastEthernet 0/0 interface and serial 0/1 of RouterB?

To answer this question, first look for clues in Figure 3.9. The first clue is that interface S0/0 on RouterA has IP address 192.168.55.2/30 assigned, which makes for an easy answer. A /30, as you know, is 255.255.255.252, which gives you a block size of 4. Your subnets are 0, 4, 8, etc. Since the known host has an IP address of 2, the only other valid host in the zero subnet is 1, so the third answer down is what you want for the s0/1 interface of RouterB.

The next clues are the listed number of hosts for each of the LANs. RouterA needs 7 hosts, a block size of 16 (/28); RouterB needs 90 hosts, a block size of 128 (/25); and RouterC needs 23 hosts, a block size of 32 (/27).

Figure 3.10 shows the answers to this question.

FIGURE

3.10

Solution to VLSM design example

192.168.55.2/30

RouterA

S0/1: 192.168.55.1/30 RouterB

RouterC

S0/0

F0/0:

F0/0:

F0/0:

192.168.55.29/28

192.168.55.132/25

192.168.55.57/27

7 hosts

90 hosts

23 hosts

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