How To Tell The Count of IP Numbers In a CIDR Notation

This post about how to get the count of IP numbers a CIDR notation depicts, is in the series of posts on Tips to Quickly Perform Binary and CIDR Operations. It is borne out of my experience working on ip-num, a typescript library for working with ASN, IPv4, and IPv6 numbers.

How to tell the count of IP number in a CIDR notation

Task: 

You have a CIDR notation and you want to quickly calculate how many ip numbers present in the range represented by the CIDR notation.

Step to answer:

• Subtract the CIDR prefix from 32. If it is an IPv6 address, subtract from 128
• Raise 2 to the power of the result of the above subtraction

Example:

How many IP addresses are in a /17, /25, /30 assuming IPv4?

/17 = 2 ^(32-17) = 2 ¹⁵ = 32768 IP addresses

/25 = 2 ^(32-25) = 2 ⁷ = 128 IP addresses

/30 = 2 ^(32-30) = 2 ² = 4 IP addresses

Previous post: How to tell the size of IP number in a CIDR notation
Next post: How to tell which CIDR notation is bigger