VLSMs (Variable-Length Subnet Masks) provide
the capability to include more than one subnet mask within a class-based
address, and the capability to subnet an already subnetted network
address. VLSMs do this by using a portion of the host address space as a
subnet address. The term variable is used because the subnet
address field can be variable length, such as two bits, three bits, or
four bits, as opposed to using a full byte for the subnet. These
capabilities offer the following two benefits:
- Even more efficient use of IP addresses
--- Without
the use of VLSMs, companies are locked into implementing a single subnet
within a NIC number in the entire network. With VLSMs, you can create a
subnet with only two hosts, for example, which is ideal for serial
links.
For example, consider that the
172.16.0.0/16 network address is divided into subnets using
172.16.0.0/24 masking and one of the subnetworks in this range,
172.16.14.0/24, is further divided into smaller subnets with the
172.16.14.0/27 masking (see the main figure).
These subnets range from 172.16.14.4 to 172.16.14.252. In the main
figure, one of the smaller subnets is further divided with the
172.16.14.128/30 subnet to be used on the WAN links and ultimately, it
avoids wasting subnets.
Greater capability to use route
summarization --- VLSMs allow for
more hierarchical levels within your addressing plan, and thus allow for
better route summarization within routing tables. In the main figure, for example, subnet 172.16.0.0/24 summarizes subnet
172.16.14.0. Subnet 172.16.0.0/24 includes all the addresses that are
further subnetted, using VLSMs, from subnets 172.16.14.0/27 and
172.16.14.128/30.
How to use route summarization is discussed
in more detail later in this chapter.
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