using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;

namespace Server
{
    class Program
    {
        public static string[] ip = null;
        public static int i = 0;
        static void Main(string[] args)
        {
            StartListener();
        }
        private const int listenPort = 11000;

        private static void StartListener()
        {
            bool done = false;

            UdpClient listener = new UdpClient(listenPort);
            IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, listenPort);

            try
            {
                while (!done)
                {
                    byte[] bytes = listener.Receive(ref groupEP);

                    if (Encoding.ASCII.GetString(bytes, 0, bytes.Length) == "Connect")
                    {
                        ip[i] = groupEP.ToString();
                        i++;
                    }
                }

            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
            finally
            {
                listener.Close();
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;

namespace Client
{
    class Program
    {
        static void Main(string[] args)
        {
            Socket s = new Socket(AddressFamily.InterNetwork, SocketType.Dgram,
            ProtocolType.Udp);

            IPAddress broadcast = IPAddress.Parse("127.0.0.1");

            byte[] sendbuf = Encoding.ASCII.GetBytes("Connect");
            IPEndPoint ep = new IPEndPoint(broadcast, 11000);

            s.SendTo(sendbuf, ep);

            Console.WriteLine("Message sent to the broadcast address");
            
        }
    }
}

ip[i] = groupEP.ToString();  i++;