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Variable-Length Subnet Masks

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 19, 2016.

Chapter Description

In this chapter fromCCENT/CCNA ICND1 100-105 Official Cert Guide, Academic Edition, author Wendell Odom introduces variable-length subnet masks (VLSM). 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.

Review Activities

Chapter Summary

  • Variable Length Subnet Masks (VLSM) simply means that the subnet design uses more than one mask in the same classful network.

  • VLSM provides many benefits for real networks, mainly related to how you allocate and use your IPv4 address space. Because a mask defines the size of the subnet (the number of host addresses in the subnet), VLSM enables engineers to better match the need for addresses with the size of the subnets.

  • VLSM can be helpful for both public and private IPv4 addresses, but the benefits are more dramatic with public networks. With public networks, the address savings help engineers avoid having to obtain another registered IPv4 network number from regional IP address assignment authorities.

  • In addition, more advanced classless routing protocols use VLSM to support manual route summarization, which enables a routing protocol to advertise one route for a larger subnet instead of multiple routes for smaller subnets.

  • To support VLSM, the routing protocol must advertise the mask along with each subnet. Without mask information, the router receiving the update would be unable to determine network address from host address.

  • By definition, classless routing protocols advertise the mask with each advertised route, and classful routing protocols do not.

  • Cisco routers do not configure VLSM, enable or disable it, or need any configuration to use it. From a configuration perspective, VLSM is simply a side effect of the ip address interface subcommand. Routers collectively configure VLSM by virtue of having IPv4 addresses in the same classful network but with different masks.

  • There are basically five steps to using VLSM. You must find all the subnet IDs that could be used, rule out the ones that would cause an overlap, and then check to see whether the requirement guides you to pick either the numerically lowest (or highest) subnet ID. This list outlines the specific steps:

    • Step 1. Pick the subnet mask (prefix length) for the new subnet, based on the design requirements (if not already listed as part of the question).

    • Step 2. Calculate all possible subnet numbers of the classful network using the mask from Step 1, along with the subnet broadcast addresses.

    • Step 3. Make a list of existing subnet IDs and matching subnet broadcast addresses.

    • Step 4. Rule out overlapping new subnets by comparing the lists from the previous two steps.

    • Step 5. Choose the new subnet ID from the remaining subnets identified at Step 4, paying attention to whether the question asks for the numerically lowest or numerically highest subnet ID.

Review Questions

  1. Which of the following routing protocols support VLSM? (Choose three answers.)

    1. RIPv1

    2. RIPv2

    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?






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 22-6 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 22-6 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 22-7 Key Topics for Chapter 22

Key Topic Element


Page Number

Table 22-1

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.”

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

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