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2.3 | ![]() |
VLSM | |
2.3.1 | ![]() |
Variable-Length Subnet Masks |
Variable-Length Subnet Mask (VLSM)
allows an organization to use more than one subnet mask within the same
network address space. Implementing VLSM is often referred to as
subnetting a subnet and it can be used to maximize addressing
efficiency.
Consider the subnets created by borrowing three bits from the host
portion of the Class C address, 207.21.24.0.
If the ip subnet-zero command is used, this mask creates seven usable subnets of 30 hosts each. Four of these subnets can be used to address remote offices at sites A, B, C, and D.![]() Unfortunately, there are only three subnets left for future growth and three point-to-point WAN links between the four sites remain to be addressed. If the three remaining subnets were assigned to the WAN links, the supply of IP addresses would be completely exhausted. This addressing scheme would also waste more than a third of the available address space. There are ways to avoid this kind of waste. Over the past 20 years, network engineers have developed three critical strategies for efficiently addressing point-to-point WAN links:
Private addresses and IP unnumbered are discussed in detail later in this module. This section focuses on VLSM. When VLSM is applied to an addressing problem, it breaks the address up into groups or subnets of various sizes. Large subnets are created for addressing LANs and very small subnets are created for WAN links and other special cases. A 30-bit mask is used to create subnets with two valid host
addresses. This is the exact number needed for a point-to-point
connection. Figure
Subnetting the 207.21.24.192/27 subnet in this way supplies
another eight ranges of addresses to be used for point-to-point
networks. For example, the network 207.21.24.192/30 can be used to
address the point-to-point serial link between the Site A router and
the Site B router.
Figure
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