IP Addressing and Subnetting Including IPv6
A clear, comprehensive guide to IP addressing, including technical details on IP message formats and standards.
TABLE OF CONTENTS IP Addressing and Subnetting Including IPv6
Chapter 5: Variable-Length Subnet Masking
Introduction
Variable-length subnet masks (VLSM) allow network administrators to "right size" each subnet. With fixed-length subnet masks, however, each subnet in the network is the same size because each device has the same subnet mask, regardless of the need for addresses in each subnet. If we select a class mask of 255.255.254.0, each subnet is allocated 510 addresses. Most LANs have an upper limit of less than 150 devices due to traffic patterns and capacity of the physical LAN media.
In actual fact, each network, WAN or LAN, has a different number of devices. With VLSM, the address administrator can more closely match the addressing needs of each subnet and use the address space more efficiently.
Why Are Variable-Length Masks Necessary?
Here is a parallel example of why VLSM is important. When restaurants sell pie, each piece of the pie is the same size. Each restaurant patron gets the same size pie slice regardless of "the need" (see Figure 5.1).
Figure 5.1: Uniform pie slices.
Ah, yes, the pie chart! If these people were subnets, they would all get the same size subnet regardless of how many addresses they really needed. Let's look at another possibility in Figure 5.2. Each person has a different appetite; they all need a different size slice of the pie.
Figure 5.2: Variable-size pie slices.
For IT Professionals: VLSM Addressing Plan
Devleoping an addressing plan based on VLSM requires complete knowledge of the network being subnetted. During the design phase of a new network, applying...
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