CCNA 200-301 Portable Command Guide, 5th Edition

Chapter 3. Variable Length Subnet Masking (VLSM)

This chapter provides information concerning the following topics:

• IP subnet zero

• VLSM example

Variable-length subnet masking (VLSM) is the more realistic way of subnetting a network to make the most efficient use of all of the bits.

Remember that when you perform classful (or what I sometimes call classical) subnetting, all subnets have the same number of hosts because they all use the same subnet mask. This leads to inefficiencies. For example, if you borrow 4 bits on a Class C network, you end up with 16 valid subnets of 14 valid hosts per subnet. A point-to-point link to another router only needs 2 hosts, but with classical subnetting, you end up wasting 12 of those hosts. Even with the ...