tcp socket error message (C#)

I'm trying to make a tcp socket. So at the momment I just want to use an example code posted on Unity Answers (http://answers.unity3d.com/questions/15422/unity-project-and-3rd-party-apps.html). using UnityEngine; using System.Collections; using System; using System.IO; using System.Net.Sockets;

 public class tcptest : MonoBehaviour {
     internal Boolean socketReady = false;
     
     TcpClient mySocket;
     NetworkStream theStream;
     StreamWriter theWriter;
     StreamReader theReader;
     String Host = "localhost";
     Int32 Port = 843;
     
     void Start () {
         setupSocket();
         writeSocket("Hello");
     }
     void Update () {
     }
 
     void OnApplicationQuit()
     {
         try
         {
             closeSocket();
         }
         catch(Exception e)
         {
             Debug.Log(e.Message);
         }
     }
     // **********************************************
     public void setupSocket() {
         try {
             mySocket = new TcpClient(Host, Port);
             theStream = mySocket.GetStream();
             theWriter = new StreamWriter(theStream);
             theReader = new StreamReader(theStream);
             socketReady = true;
         }
         catch (Exception e) {
             Debug.Log("Socket error: " + e);
         }
     }
 
     public void writeSocket(string theLine) {
         if (!socketReady)
             return;
         String foo = theLine + "\r\n";
         theWriter.Write(foo);
         theWriter.Flush();
     }
     public String readSocket() {
         if (!socketReady)
             return "";
         if (theStream.DataAvailable)
             return theReader.ReadLine();
         return "";
     }
     public void closeSocket() {
         if (!socketReady)
             return;
         theWriter.Close();
         theReader.Close();
         mySocket.Close();
         socketReady = false;
     }
 } // end class s_TCP

But the thing is: I allways get an error message: "System.Net.Sockets.SocketException: No connection could be made because the target machine actively refused it."

What is the problem and how can I fix it ?