VLSM - variable length subnet mask (Step by step), Subnetting - According to the hosts/subnet
In VLSM subnetting network administrators can divide an ip address space into subnets of different sizes and allocate it according to the individual need on a network.
This video gives two examples on vlsm subnetting and how to make it easier.
In vlsm subnetting different subnets can be of different sizes.