C# – How to ping faster when I reach unreachable IP

I have an app which pings IP or IP range. The problem is that when hosts are closed it takes longer to ping than they are open. When host is closed the time to ping is about 1-2 seconds.

How could I make it faster when hosts are closed?

This is my code:

using System;
using System.Text;
using System.Windows.Forms;
using System.Net.NetworkInformation;

namespace Range_Pinger
{
    public partial class PingIPRange : Form
    {
        uint startIP, endIP, currentIP;
        int count = 0;
        int open = 0;
        int closed = 0;

        public PingIPRange()
        {
            InitializeComponent();

            tmrPingInterval.Tick += new EventHandler(tmrPingInterval_Tick);
        }

        void tmrPingInterval_Tick(object sender, EventArgs e)
        {
            if (txtTo.Text == string.Empty) Ping(ip2str(startIP));
            else
            {
                if (currentIP >= endIP) tmrPingInterval.Stop();
                Ping(ip2str(currentIP));
                currentIP++;
            }

            count++;

            tsslPingCount.Text = "Total number of pings: " + count.ToString() + 
                " Open IPs: " + open.ToString() + " Closed IPs: " + closed.ToString();
        }

        static uint str2ip(string ip)
        {
            string[] numbers = ip.Split('.');

            uint x1 = (uint)(Convert.ToByte(numbers[0]) << 24);
            uint x2 = (uint)(Convert.ToByte(numbers[1]) << 16);
            uint x3 = (uint)(Convert.ToByte(numbers[2]) << 8);
            uint x4 = (uint)(Convert.ToByte(numbers[3]));

            return x1 + x2 + x3 + x4;
        }

        static string ip2str(uint ip)
        {
            string s1 = ((ip & 0xff000000) >> 24).ToString() + ".";
            string s2 = ((ip & 0x00ff0000) >> 16).ToString() + ".";
            string s3 = ((ip & 0x0000ff00) >> 8).ToString() + ".";
            string s4 = (ip & 0x000000ff).ToString();

            return s1 + s2 + s3 + s4;
        }


        private void btnPing_Click(object sender, EventArgs e)
        {
            txtDisplay.Text = string.Empty;
            tsslPingCount.Text = string.Empty; 
            count = 0;
            open = 0;
            closed = 0;
            tmrPingInterval.Interval = int.Parse(nudInterval.Value.ToString());

            try
            {
                startIP = str2ip(txtFrom.Text);
                if (txtTo.Text != string.Empty) endIP = str2ip(txtTo.Text);
                currentIP = startIP;
                tmrPingInterval.Start();
            }
            catch
            {
                MessageBox.Show("Invalid input. It must be something like: 255.255.255.255");
            }
        }

        private void btnStop_Click(object sender, EventArgs e)
        {
            tmrPingInterval.Stop();
        }

        private void Ping(string address)
        {
            Ping pingSender = new Ping();
            PingOptions options = new PingOptions();
            options.DontFragment = true;
            string data = "01234567890123456789012345678901";
            byte[] buffer = Encoding.ASCII.GetBytes(data);
            int timeout = 120;
            try
            {
                PingReply reply = pingSender.Send(address, timeout, buffer, options) ;
                if (reply.Status == IPStatus.Success)
                {
                    open++;
                    txtDisplay.AppendText("Host " + address + " is open." + Environment.NewLine);
                }
                else
                {
                    closed++;
                    txtDisplay.AppendText("Host " + address + " is closed." + Environment.NewLine);
                }
            }
            catch (Exception ex)
            {
                txtDisplay.SelectedText += Environment.NewLine + ex.Message;
            }
        }

        private void tsmiExit_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}

This is what I have now:

    [DllImport("iphlpapi.dll", ExactSpelling = true)]
    public static extern int SendARP(IPAddress DestIP, int SrcIP, byte[] pMacAddr, ref uint PhyAddrLen);

    private void Ping(IPAddress address)
    {
        byte[] macAddr = new byte[6];
        uint macAddrLen = (uint)macAddr.Length;

        if (SendARP(address, 0, macAddr, ref macAddrLen) == 0)
        {
            txtDisplay.AppendText("Host " + address + " is open." + Environment.NewLine);
        }
        else txtDisplay.AppendText("Host " + address + " is closed." + Environment.NewLine);
    }