Calculate number of subnets of a given size you can 'create' or 'take' or 'extract' from a given network.

program.cs

using System;
					
public class Program
{
	public static void Main()
	{
		// Example: How many /30's can we fit in a /28?
		var sourcePrefixLength = 28;
		var destinationPrefixLength = 30;
		
		Console.WriteLine(CalculateNumberOfSubnets(sourcePrefixLength, destinationPrefixLength));	
	}
	
	// This method works for IPv4 and IPv6
	public static UInt128 CalculateNumberOfSubnets(int sourcePrefixLength, int destinationPrefixLength)
		=> sourcePrefixLength > destinationPrefixLength             ? throw new Exception("Source prefix length must be less than, or equal to, destination prefix length")
		: sourcePrefixLength > 128 || destinationPrefixLength > 128 ? throw new Exception("Prefix length cannot be greater than 128")
		: sourcePrefixLength < 0 || destinationPrefixLength < 0     ? throw new Exception("Prefix length cannot be negative")
		: destinationPrefixLength - sourcePrefixLength == 128       ? UInt128.MaxValue // Handle edge case
		: (UInt128)1 << destinationPrefixLength - sourcePrefixLength;
}