Work out subnet split (vlsm) instantly with clear inputs, formula shown and shareable results.
Variable length subnet masking sizes each subnet to its own host count instead of forcing one mask on the whole network. Round the host requirement up to the next power of two after adding the network and broadcast addresses, and the number of host bits gives the child prefix directly. Comparing subnets needed against subnets available tells you whether the parent block is big enough.
VLSM child prefix
host bits = ceil(log2(hosts + 2)); child prefix = 32 - host bits; children = 2^(child prefix - base prefix)
Because the network and broadcast addresses are not assignable, a subnet that must hold 510 hosts needs 512 addresses, which is 9 host bits and therefore a /23.
Allocate the largest subnets first from the top of the block. Filling large requirements after small ones fragments the space and forces you to waste an aligned block.