ICND2 – Chapter 5 – VLSM – Variable Length Subnet Masks

Chapter 5 was a very short and fun chapter. Over 1/2 of the chapter though was devoted to the perils of VLSM overlap and how to calculate and determine if you have created overlap. Bottom line is that you should design so as to not overlap subnets. Odom used submarines in his diagrams for this chapter which was pretty cool. No Fred, Yosemite, or Albuquerque in this chapter. 🙂 Here are my notes:

VLSM occurs when a network uses more than one mask for different subnets of a single classful network.

For example, if you take a (Class C network) and divide it up into a bunch of /27s, /28s, and /30s then you are using VLSM.

Using all /30s with the same Class C block is NOT VLSM but rather simply subnetting the network.

By default, classless routing protocols advertise the subnet mask with the route. Classful routing protocols (RIPv1 and IGRP) do not.

When planning to subnet a block with VLSM, assign your largest blocks first and then work to the smallest size blocks.

VLSM Overlap – Don’t Do It! 🙂


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