How do you perform VLSM subnetting step by step?

Allocate largest-first: carve the biggest block off the top, then start each next subnet at the address right after the previous block's broadcast, shrinking the mask as host needs shrink.

VLSM (Variable Length Subnet Masking) subnetting is a repeatable routine. First, rank your segments by host count, biggest to smallest. For each one in turn, pick the smallest prefix whose 2^h − 2 covers its hosts (a 100-host LAN needs /25, a 50-host LAN /26, a 2-host WAN link /30). Then place it: the first block starts at the base network address, and every later block starts at the address immediately after the previous block's broadcast, so they pack together with no gaps. Because you go largest-first, each big block lands on a correctly aligned boundary before the smaller, more flexible blocks fill in behind it. The mask gets longer (the block shorter) as you move down the list, which is the whole point of VLSM — you spend exactly as many addresses as each segment needs instead of giving every subnet the same fixed size.

VLSM Subnetting Example:

Given: 192.168.20.0/24 Requirements:

• Building A: 100 hosts → /25 (126 hosts)
• Building B: 50 hosts → /26 (62 hosts)
• Building C: 25 hosts → /27 (30 hosts)
• WAN links: 2 hosts each → /30 (2 hosts)

Step-by-Step Allocation (largest first):

Critical rule: Always start the next subnet at the first address AFTER the previous subnet's broadcast address.

Go deeper:

• FLSM and VLSM, worked example (PracticalNetworking) — the same largest-first routine applied step by step.
• Visual Subnet Calculator (davidc.net) — reproduce the 192.168.20.0/24 allocation block by block.