using System;

using System.Net;

​

public class Program

{

    public static void Main()

    {

        uint intSubnetMask = ConvertToSubnet("10.0.128.0/19");

        string strSubNetIP = toip(intSubnetMask);

        bool blnResult = IsAddressOnSubnet("10.0.130.44", "10.0.128.0", strSubNetIP);

        Console.WriteLine("Hello World : " + blnResult.ToString());

    }

​

    static bool IsAddressOnSubnet(string address, string subnet, string mask)

    {

        try

        {

            IPAddress Address = IPAddress.Parse(address);

            IPAddress Subnet = IPAddress.Parse(subnet);

            IPAddress Mask = IPAddress.Parse(mask);

            Byte[] addressOctets = Address.GetAddressBytes();

            Byte[] subnetOctets = Mask.GetAddressBytes();

            Byte[] networkOctets = Subnet.GetAddressBytes();

            return ((networkOctets[0] & subnetOctets[0]) == (addressOctets[0] & subnetOctets[0]))

                && ((networkOctets[1] & subnetOctets[1]) == (addressOctets[1] & subnetOctets[1])) 

                && ((networkOctets[2] & subnetOctets[2]) == (addressOctets[2] & subnetOctets[2])) 

                && ((networkOctets[3] & subnetOctets[3]) == (addressOctets[3] & subnetOctets[3]));

        }

        catch (System.Exception ex)

        {

            return false;

        }

    }

​

    static uint ConvertToSubnet(string IP)

    {

        string[] parts = IP.Split('.', '/');

        uint ipnum = (Convert.ToUInt32(parts[0]) << 24) | (Convert.ToUInt32(parts[1]) << 16) | (Convert.ToUInt32(parts[2]) << 8) | Convert.ToUInt32(parts[3]);

        int maskbits = Convert.ToInt32(parts[4]);

        uint mask = 0xffffffff;

        mask <<= (32 - maskbits);

        return mask;

    }

​

    static string toip(uint ip)

    {

        return String.Format("{0}.{1}.{2}.{3}", ip >> 24, (ip >> 16) & 0xff, (ip >> 8) & 0xff, ip & 0xff);

    }

}