Subnetting is possible based on number of bits burrowing from hosts portion. For example, if we have 8 bits allocated to host portion for /24 networks then we have 8 bits to work on so it depends how many subnetting we need to do. If we need to divide it to 2 subnets we need to burrow 1 bit. Formula is 2^n(number of bits burrowed from host portion).