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CCNA ICND2 Cert Guide: Variable Length Subnet Masks

This chapter introduces VLSM, including VLSM Concepts and Configuration, Finding VLSM Overlaps, Adding New Subnets to an Existing VLSM Design, and Designing a Subnetting Plan Using VLSM.
This chapter is from the book

VLSM Concepts and Configuration: This section explains the issues and solutions when designing an internetwork that uses VLSM.

Finding VLSM Overlaps: This section is the first of three that focus on applying VLSM concepts in a particular way. In this case, it focuses on analyzing a deployed internetwork to find cases in which the subnets’ address ranges overlap, which causes IP routing problems.

Adding New Subnets to an Existing VLSM Design: This section examines how to choose new subnets, based on an existing design plus the requirements for the new subnets. This section emphasizes how to avoid mistakenly choosing subnets that overlap.

Designing a Subnetting Plan Using VLSM: This section discusses cases in which you start with no design at all, but instead with a set of requirements and an IP network. Your job: choose a number of masks, the number of subnets that use each mask, and the specific subnet IDs to use with each mask.

Most of the IP addresses and subnetting content sits inside the ICND1 part of the CCNA puzzle. This chapter explores the one pure addressing topic in the ICND2 part of the mix: variable length subnet masks (VLSM).

VLSM builds on the subnetting concepts in ICND1. If you have a good handle on those details, great! If you are still a little unsure, it may be a good time to review and practice subnetting. For instance, to do some of the exercises in this chapter, you need to remember how and why you would pick a particular mask, given the need for a subnet to support some number of host IP addresses. You also need to be able to find all the subnet IDs of a single classful network when using a single mask. Using both sets of skills, this chapter expands on those concepts when using multiple masks. Look at this chapter as an opportunity to learn VLSM, as well as to review and strengthen your subnetting skills.

“Do I Know This Already?” Quiz

The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these six self-assessment questions, you might want to move ahead to the section, “Exam Preparation Tasks.” Table 5-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those headings so that you can assess your knowledge of these specific areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes.”

Table 5-1. “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

Foundations Topics Section


VLSM Concepts and Configuration

1, 2

Finding VLSM Overlaps

3, 4

Adding a New Subnet to an Existing VLSM Design


Designing a Subnetting Plan Using VLSM


  1. Which of the following routing protocols support VLSM?

    1. RIP-1
    2. RIP-2
    3. EIGRP
    4. OSPF
  2. What does the acronym VLSM stand for?

    1. Variable length subnet mask
    2. Very long subnet mask
    3. Vociferous longitudinal subnet mask
    4. Vector-length subnet mask
    5. Vector loop subnet mask
  3. R1 has configured interface Fa0/0 with the ip address command. Which of the following subnets, when configured on another interface on R1, would not be considered an overlapping VLSM subnet?

  4. R4 has a connected route for Which of the following answers lists a subnet that overlaps with this subnet?

  5. A design already includes subnets,, and Which of the following subnets is the numerically lowest subnet ID that could be added to the design, if you wanted to add a subnet that uses a /28 mask?

  6. An engineer is following a VLSM design process of allocating the largest subnets first, as the numerically lowest subnets, and then subdividing the next subnet into smaller pieces for the next smaller size of subnet. In this case, the engineer has reserved the first three /20 subnets of to be used in an internetwork:,, and The next smaller size subnets to be allocated will be subnets with mask /25; this design requires 10 such subnets. Assuming the engineer continues to allocate subnets in sequence, which answers lists the tenth of these /25 subnets?


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