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Variable-Length Subnet Masks

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.

Chapter Review

One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book, DVD, or interactive tools for the same material found on the book’s companion website. Refer to the “Your Study Plan” element for more details. Table 22-7 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.

Table 22-7 Chapter Review Tracking

Review Element

Review Date(s)

Resource Used

Review key topics


Book, DVD/website

Review key terms


Book, DVD/website

Repeat DIKTA questions


Book, PCPT

Review memory tables


Book, DVD/website

Practice finding VLSM overlaps


DVD Appendix H, DVD/website

Practice adding new VLSM subnets


DVD Appendix H, DVD/website

Review All the Key Topics


Table 22-8 Key Topics for Chapter 22

Key Topic Element


Page Number

Table 22-2

Classless and classful routing protocols listed and compared



Rule about subnetting designs cannot allow subnets to overlap



Steps to analyze an existing design to discover any VLSM overlaps



Steps to follow when adding a new subnet to an existing VLSM design


Key Terms You Should Know

  • classful routing protocol

  • classless routing protocol

  • overlapping subnets

  • variable-length

  • subnet masks (VLSM)

Additional Practice for This Chapter’s Processes

For additional practice with finding VLSM overlaps and adding a new subnet to a VLSM design, you may do the same set of practice problems using your choice of tools:

  • Application: Use the Variable-Length Subnet Masks application on the DVD or companion website.

  • PDF: Alternatively, practice the same problems found in both these apps using DVD Appendix H, “Practice for Chapter 22: Variable-Length Subnet Masks.”

4. Answers to Earlier Practice Problems | Next Section Previous Section

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