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