Subnet Mask and CIDR: How many valid hosts can be used on a specific subnet (Classless Addressing)?

CIDR or Classless Inter-Domain Routing another IP addressing system that allows more flexible allocation of IP addresses compared to the original Classful addressing scheme (A, B, C, D and E). The Classful IP addressing scheme can be wasteful. For example, giving the minimum IP address allocation of 256 (Class C) to a network which only requires 15 addresses. 

The table below shows us how many available addresses are there in subnet for a specific CIDR notation. 

• 2^(32-CIDR)= number of devices on the local network
• Number of valid host is always two less than the subnet block

http://community.engeniustech.com/topic/1024-subnet-mask-and-cidr-how-many-valid-hosts-can-be-used-on-a-specific-subnet-classless-addressing/