Chapter4.Variable-Length Subnet Masks

Chapter 4. Variable-Length Subnet Masks

The preceding chapter examined the powerful innovation known as subnetting in its original form: Fixed-Length Subnet Masking (FLSM). At its introduction, FLSM was called simply subnetting. By any name, it was a revolutionary and necessary evolution of the IP address architecture that enabled a tremendous reduction in the waste of IP addresses. With the benefit of hindsight, we can see that FLSM was but a half step in the right direction. Its single greatest benefit was that it validated the concept of borrowing bits from the host field of an IP address to create locally significant subnetwork identification addresses. But the simplifying assumption of permitting just one subnet mask for all subnets created from a network address proved to be both unnecessary and inefficient.

In reality, subnets are hardly ever of the same approximate size. Consequently, FLSM's one-size-fits-all design philosophy created a substantial number of wasted addresses. Solving this conundrum was easy: Permit the creation of variable-length subnets. In theory, this would enable subnets to be created more efficiently by making each subnet mask specifically tailored to each subnet.

To help make this esoteric concept a bit more real, I'll use a sample network to show you how subnetting works mathematically. Throughout this chapter, we'll build on this sample network and look at some interesting things, including practical implications. We'll also look at the challenges of managing an address space subnetted into flexibly sized subnets using a technique known as Variable-Length Subnet Masking.