How to subtract an IP address? ( both IPv4 and IPv6)

I have a requirement to get the number of IP addresses in an IP range identified by a startIP and an endIP both for IPv4 and IPv6 ranges.

Can anyone point to ways that can be used to achieve the subtraction of IP addresses?

The number of IP addresses will be = endIP - startIP + 1

Any responses are highly appreciated.

How's this?

class Program
{

    static void Main(string[] args)
    {
        IPAddress a = new IPAddress(new byte[] { 192, 168, 11, 12 });
        IPAddress b = new IPAddress(new byte[] { 192, 168, 12, 12 });
        long diff = Difference(a, b);
    }

    private static Int64 ConvertToLong(IPAddress a)
    {
        byte[] addressBits = a.GetAddressBytes();
        Int64 retval = 0;
        for (int i = 0; i < addressBits.Length; i++)
        {
            retval = (retval << 8) + (int)addressBits[i];
        }

        return retval;
    }

    private static Int64 Difference(IPAddress a, IPAddress b)
    {
        return Math.Abs(ConvertToLong(a) - ConvertToLong(b)) - 1;
    }

}

class Program          
{  
static void Main(string[] args)   
    {   
        string a = "192.168.11.12";
        string b = "192.168.12.12";
        int diff = Math.Abs(IPToInt(a) - IPToInt(b)) + 1;
    } 
int IPToInt(string IP) 
{ 
    return IPAddress.NetworkToHostOrder(BitConverter.ToInt32(IPAddress.Parse(IP).GetAddressBytes(), 0)); 
}       
}