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Variable-Length Subnet Masks

Contents

  1. "Do I Know This Already?" Quiz
  2. Foundation Topics
  3. Chapter Review
  4. Answers to Earlier Practice Problems

Chapter Description

In this sample chapter from CCENT/CCNA ICND1 100-105 Official Cert Guide, author Wendell Odom introduces variable-length subnet masks (VLSM)

Variable-length subnet masks (VLSM) simply means that the subnet design uses more than one mask in the same classful network. VLSM has some advantages and disadvantages, but when learning, the main challenge is that a subnetting design that uses VLSM requires more math, and it requires that you think about some other issues as well. This chapter walks you through the concepts, the issues, and the math.

Answers to Earlier Practice Problems

Answers to Practice Finding VLSM Overlaps

This section lists the answers to the three practice problems in the section “Practice Finding VLSM Overlaps,” as listed earlier in Table 22-4. Note that the tables that list details of the answer reordered the subnets as part of the process.

In Problem 1, the second and third subnet IDs listed in Table 22-9 happen to overlap. The second subnet’s range completely includes the range of addresses in the third subnet.

Table 22-9 VLSM Overlap Problem 1 Answers (Overlaps Highlighted)

Reference

Original Address and Mask

Subnet ID

Broadcast Address

1

10.1.1.1/20

01.1.0.0

10.1.15.255

2

10.1.17.1/21

10.1.16.0

10.1.23.255

3

10.1.23.254/22

10.1.20.0

10.1.23.255

4

10.1.29.101/23

10.1.28.0

10.1.29.255

5

10.1.34.9/22

10.1.32.0

10.1.35.255

In Problem 2, again the second and third subnet IDs (listed in Table 22-10) happen to overlap, and again, the second subnet’s range completely includes the range of addresses in the third subnet. Also, the second and third subnet IDs are the same value, so the overlap is more obvious.

Table 22-10 VLSM Overlap Problem 2 Answers (Overlaps Highlighted)

Reference

Original Address and Mask

Subnet ID

Broadcast Address

1

172.16.122.1/30

172.16.122.0

172.16.122.3

2

172.16.122.57/27

172.16.122.32

172.16.122.63

3

172.16.122.33/30

172.16.122.32

172.16.122.35

4

172.16.126.151/22

172.16.124.0

172.16.127.255

5

172.16.128.151/20

172.16.128.0

172.16.143.255

In Problem 3, three subnets overlap. Subnet 1’s range completely includes the range of addresses in the second and third subnets, as shown in Table 22-11. Note that the second and third subnets do not overlap with each other, so for the process in this book to find all the overlaps, after you find that the first two subnets overlap, you should compare the next entry in the table (3) with both of the two known-to-overlap entries (1 and 2).

Table 22-11 VLSM Overlap Problem 3 Answers (Overlaps Highlighted)

Reference

Original Address and Mask

Subnet ID

Broadcast Address

1

192.168.1.113/28

192.168.1.112

192.168.1.127

2

192.168.1.122/30

192.168.1.120

192.168.1.123

3

192.168.1.125/30

192.168.1.124

192.168.1.127

4

192.168.1.245/29

192.168.1.240

192.168.1.247

5

192.168.1.253/30

192.168.1.252

192.168.1.255

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