C#中使用Udp协议实现网络聊天的问题

public partial class Form1 : Form
    {
       private UdpClient receiveUdpClient;    
       private UdpClient sendUdpClient;   
       private const int port = 80;  
       IPAddress ip;   
       IPAddress remoteIp;  
        public Form1()
        {
            InitializeComponent();
            //获取本机可用IP地址   
            ip = Dns.GetHostAddresses(Dns.GetHostName())[0];
            remoteIp=ip; //默认远程主机地址
          //remoteIp = new IPAddress(new byte[]{222,25,180,15});//已设置好远程主机ip，用于局域网测试
           textBoxRemoteIP.Text = remoteIp.ToString();  
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            
            //创建一个线程接收远程主机发来的信息   
            Thread myThread = new Thread(ReceiveData);
            //将线程设为后台运行   
            myThread.IsBackground = true;
            myThread.Start();
            textBoxSend.Focus();
        }  
        private void ReceiveData()  
        {  
            IPEndPoint local = new IPEndPoint(ip, port);  
            receiveUdpClient = new UdpClient(local);  
            IPEndPoint remote = new IPEndPoint(IPAddress.Any, 0);  
            while (true)  
            {  
                try  
                {  
                   byte[] receiveBytes = receiveUdpClient.Receive(ref remote);  
                    string receiveMessage = Encoding.Unicode.GetString(receiveBytes, 0, receiveBytes.Length);  
                   AddItem(listBoxReceive, string.Format("来自{0}：{1}", remote, receiveMessage));  
               }  
               catch  
                {  
                   break;  
                }  
           }  
        }
        //发送信息
        private void button1_Click(object sender, EventArgs e)       
        {
            Thread t = new Thread(SendMessage);
            t.IsBackground = true;
            t.Start(textBoxSend.Text);  
        }
         private void SendMessage(object obj)  
        {  
           string message = (string)obj;  
            sendUdpClient = new UdpClient(0);  
            byte[] bytes = System.Text.Encoding.Unicode.GetBytes(message);  
            IPEndPoint iep = new IPEndPoint(ip, port);  
            try  
           {  
               sendUdpClient.Send(bytes, bytes.Length, iep);  //运行至此处跳转到catch，出错
               AddItem(listBoxStatus, string.Format("向{0}发送：{1}", iep, message));  
                ClearTextBox();  
            }  
          catch (Exception ex)  
            {  
               AddItem(listBoxStatus, "发送出错:" + ex.Message);  
            }  
        }//下面的代码是委托类，封装，应该没问题的  
        delegate void AddListBoxItemDelegate(ListBox listbox, string text);  
        private void AddItem(ListBox listbox, string text)  
        {  
            if (listbox.InvokeRequired)  
            {  
                AddListBoxItemDelegate d = AddItem;  
                listbox.Invoke(d, new object[] { listbox, text });  
            }  
           else  
           {  
               listbox.Items.Add(text);  
                listbox.SelectedIndex = listbox.Items.Count - 1;  
                listbox.ClearSelected();  
            }  
        }  
       delegate void ClearTextBoxDelegate();  
        private void ClearTextBox()  
       {  
            if (textBoxSend.InvokeRequired)  
            {  
               ClearTextBoxDelegate d = ClearTextBox;  
                textBoxSend.Invoke(d);  
            }  
            else  
          {  
              textBoxSend.Clear();  
               textBoxSend.Focus();  
          }  
       }
    }
}
同一台电脑上运行：  
在局域网测试过了，另外一台机器上能接收到我发送的信息，而我不能接收到对面发送的信息