Practical Networking .net

FLSM and VLSM – Part 1

After many requests to extend the Subnetting Mastery video series, I am adding an appendix which will cover Fixed Length Subnet Masks (FLSM) and Variable Length Subnet Masks (VLSM).

FLSM - Subnetting Mastery

This is Part 1, which teaches how to answer questions of this variety:

  • If you start with a /18, what size sub-network would you need to create 100 sub-networks?
  • If you start with a /21, how many sub-networks could you create that could contain 50 IP addresses?

In this video, we’ll teach you about the Network Doubling logic, and how to simplify the questions above, and easily come to the correct answer. Then we’ll discuss the difference between FLSM questions and VLSM questions.

Finally, we’ll show you 2N notation, which uses a Powers of 2 table to quickly answer questions like those above.

The next video in the series will dive deeper in to VLSM.

Every video in the Subnet Mastery series as well as an IPv4 Subnetting Problem Generator is available here:



  1. Garrett H. says

    Thanks so much for doing this! The clarity with which you explain these difficult topics is much appreciated.

  2. Andrew Diamantis says

    Great stuff. I would have saved a bunch of time just watching your subnetting vids right off the bat. Looking forward to your subnetting continuation on VLSM. Thanks again.

  3. Joshua Earhart says

    Better explanation and detail than my college professor, wouldn’t have learned submitting without your videos and CIDR cheatsheet..

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