using System;

​

public class Program

{

    public static void Main()

    {

        System.Net.IPAddress someIP = System.Net.IPAddress.Parse("192.168.1.23");

        System.Net.IPAddress someMASK = System.Net.IPAddress.Parse("255.255.255.128");

​

        long ipL = IPtoLong(someIP);

        long maskL = IPtoLong(someMASK);

​

        // Convert  Mask to CIDR(1-30)

        long oneBit = 0x80000000L;

        int CIDR = 0;

​

        for (int x = 31; x >= 0; x += -1)

        {

            if ((maskL & oneBit) == oneBit)

                CIDR += 1;

            else

                break;

            oneBit = oneBit >> 1;

        }

​

        string answer = LongToIp(ipL & maskL) + "/" + CIDR.ToString();

        Console.Out.WriteLine("woah woah we woah " + answer);

    }

    public static long IPtoLong(System.Net.IPAddress theIP) // convert IP to number

    {

        byte[] IPb = theIP.GetAddressBytes(); // get the octets

        long addr = 0; // accumulator for address

​

        for (int x = 0; x <= 3; x++) {

            addr |=  (System.Convert.ToInt64(IPb[x]) << (3 - x) * 8);

        }

        return addr;

    }

​

    public static string LongToIp(long theIP) // convert number back to IP

    {

        byte[] IPb = new byte[4]; // 4 octets

        string addr = ""; // accumulator for address

​

        long mask8 = MaskFromCidr(8); // create eight bit mask

​

        for (var x = 0; x <= 3; x++) // get the octets

        {

            IPb[x] = System.Convert.ToByte((theIP & mask8) >> ((3 - x) * 8));

            mask8 = mask8 >> 8;

            addr += IPb[x].ToString() + "."; // add current octet to string

        }

        return addr.TrimEnd('.');

    }

​

    private static long MaskFromCidr(int CIDR)

    {

        return  System.Convert.ToInt64(Math.Pow(2, ((32 - CIDR))) - 1) ^ 4294967295L;

    }

}

​