VLSM Concepts and Configuration
VLSM occurs when an internetwork uses more than one mask for different subnets of a single Class A, B, or C network. Figure 20-1 shows an example of VLSM used in Class A network 10.0.0.0.
Figure 20-1 VLSM in Network 10.0.0.0: Masks /24 and /30
Figure 20-1 shows a typical choice of using a /30 prefix (mask 255.255.255.252) on point-to-point serial links, with mask /24 (255.255.255.0) on the LAN subnets. All subnets are of Class A network 10.0.0.0, with two masks being used, therefore meeting the definition of VLSM.
Oddly enough, a common mistake occurs when people think that VLSM means “using more than one mask ...
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