VLSM subnet planner
Give it a base block and a list of how many hosts each subnet needs — it allocates the smallest subnet that fits each, largest first, with no overlaps or wasted gaps.
What is VLSM subnet planner?
Variable Length Subnet Masking (VLSM) is how real networks avoid wasting address space: instead of splitting a block into equal-sized subnets, each subnet is sized to fit exactly how many hosts it actually needs.
How to use it
Give it a base block and a list of host counts, one per line. It allocates the smallest subnet that fits each requirement, largest first, packing them with no gaps or overlaps — the same approach taught in networking courses and used by real ISPs and enterprises.
Example
192.168.1.0/24 split for subnets needing 100, 50, 25 and 10 hosts:
100 -> 192.168.1.0/25 50 -> 192.168.1.128/26
25 -> 192.168.1.192/27 10 -> 192.168.1.224/28