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IP Variable Length Subnet Masking (VLSM)
(Page 2 of 3)
The Solution: Variable Length Subnet Masking
The solution to this situation is
an enhancement to the basic subnet addressing scheme called Variable
Length Subnet Masking (VLSM). VLSM seems complicated at first, but
is easy to comprehend if you understand basic subnetting. The idea is
that you subnet the network, and then subnet the subnets just the way
you originally subnetted the network. In fact, you can do this multiple
times, creating subnets of subnets of subnets, as many times as you
need (subject to how many bits you have in the host ID of your address
block). It is possible to choose to apply this multiple-level splitting
to only some of the subnets, allowing you to selectively cut the "IP
address pie" so that some of the slices are bigger than others. This
means that our example company could create six subnets to match the
needs of its networks, as shown in Figure 71.
Figure 71: Class C (/24) Network Split Using Variable Length Subnet Masking (VLSM)
Using VLSM, an organization can divide its IP network multiple times, to create subnets that much better match the size requirements of its physical networks. Contrast to Figure 70.
Key Concept: Variable Length Subnet Masking (VLSM) is a technique where subnetting is performed multiple times in iteration, to allow a network to be divided into a hierarchy of subnetworks that vary in size. This allows an organization to much better match the size of its subnets to the requirements of its networks.
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Version 3.0 - Version Date: September 20, 2005
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