Variable-Length Subnet Masking lets you fit subnets of different sizes inside one address block without waste. This chapter walks through the standard 'biggest-first' planning method with a worked enterprise example.
Real organizations need subnets of very different sizes. A user LAN may need 200 hosts, a printer VLAN may need 14, a serial link needs 2. Allocating /24 to all of them wastes thousands of addresses. VLSM lets every subnet take exactly the prefix it requires.
Always allocate the largest subnet first, then the next largest, and so on. If you allocate small subnets first you fragment the block and end up unable to fit the larger ones later.
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