find net mask

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;

public class FindMask
{
    public static void Main()
    {
        byte[] data = new byte[1024];
        int recv;
        Socket host = new Socket(AddressFamily.InterNetwork, SocketType.Raw, ProtocolType.Icmp);
        IPEndPoint iep = new IPEndPoint(IPAddress.Broadcast, 0);
        EndPoint ep = (EndPoint)iep;
        ICMP packet = new ICMP();

        packet.Type = 0x11;
        packet.Code = 0x00;
        packet.Checksum = 0;
        Buffer.BlockCopy(BitConverter.GetBytes(1), 0, packet.Message, 0, 2);
        Buffer.BlockCopy(BitConverter.GetBytes(1), 0, packet.Message, 2, 2);
        Buffer.BlockCopy(BitConverter.GetBytes(0), 0, packet.Message, 4, 4);
        packet.MessageSize = 8;
        int packetsize = packet.MessageSize + 4;

        UInt16 chksm = packet.getChecksum();
        packet.Checksum = chksm;

        host.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 3000);
        host.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.Broadcast, 1);
        host.SendTo(packet.getBytes(), packetsize, SocketFlags.None, iep);
        try
        {
            data = new byte[1024];
            recv = host.ReceiveFrom(data, ref ep);
        }
        catch (SocketException)
        {
            Console.WriteLine("Unable to determine subnet mask for this subnet");
            return;
        }
        ICMP response = new ICMP(data, recv);
        Console.WriteLine("Received ICMP Type {0} packet", response.Type);
        long answer = BitConverter.ToUInt32(response.Message, 4);
        IPAddress netmask = new IPAddress(answer);
        Console.WriteLine("The subnet mask for this subnet is: {0}", netmask.ToString());
    }
}
class ICMP
{
    public byte Type;
    public byte Code;
    public UInt16 Checksum;
    public int MessageSize;
    public byte[] Message = new byte[1024];

    public ICMP()
    {
    }

    public ICMP(byte[] data, int size)
    {
        Type = data[20];
        Code = data[21];
        Checksum = BitConverter.ToUInt16(data, 22);
        MessageSize = size - 24;
        Buffer.BlockCopy(data, 24, Message, 0, MessageSize);
    }

    public byte[] getBytes()
    {
        byte[] data = new byte[MessageSize + 9];
        Buffer.BlockCopy(BitConverter.GetBytes(Type), 0, data, 0, 1);
        Buffer.BlockCopy(BitConverter.GetBytes(Code), 0, data, 1, 1);
        Buffer.BlockCopy(BitConverter.GetBytes(Checksum), 0, data, 2, 2);
        Buffer.BlockCopy(Message, 0, data, 4, MessageSize);
        return data;
    }

    public UInt16 getChecksum()
    {
        UInt32 chcksm = 0;
        byte[] data = getBytes();
        int packetsize = MessageSize + 8;
        int index = 0;

        while (index < packetsize)
        {
            chcksm += Convert.ToUInt32(BitConverter.ToUInt16(data, index));
            index += 2;
        }
        chcksm = (chcksm >> 16) + (chcksm & 0xffff);
        chcksm += (chcksm >> 16);
        return (UInt16)(~chcksm);
    }
}