What is VLSM?
Variable-length subnet masking allocates different-sized subnets inside a parent network. For every host requirement, the planner chooses the smallest conventional subnet that can hold the requested usable hosts, accounting for the network and broadcast addresses.
Worked example
50 usable hosts require at least 52 total addresses, so the next power of two is 64: a /26 subnet with 62 usable hosts. 25 hosts need a /27, 10 need a /28 and 5 need a /29.
Subnets are aligned on valid boundaries and never overlap. Review your allocation plan before applying it to production routers or firewalls.