This is one of the most common application scenario that can be found all over the net. and I'm not asking any questions about the java codes that I did because I was successful in running it on my laptop where both the client and server part of the .java file resides. Rather I have had problem getting it to work in between two computers. I tried establishing physical connection using cross-over cable to connect two computers, and did a test to see if file transfers successfully and it did, however, keeping one Server part of the .java file in one computer and client part in the other, I tried to run the server first and then the client but it got a "access denied" error. For reference here's my two .java files: /* ChatClient.java */ import java.io.BufferedReader; import java.io.InputStreamReader; import java.io.IOException; import java.io.PrintWriter; import java.net.Socket; import java.net.UnknownHostException; public class ChatClient { private static int port = 5000; /* port to connect to */ private static String host = "localhost"; /* host to connect to (server's IP)*/ private static BufferedReader stdIn; private static String nick; /** * Read in a nickname from stdin and attempt to authenticate with the * server by sending a NICK command to @out. If the response from @in * is not equal to "OK" go bacl and read a nickname again */ private static String getNick(BufferedReader in, PrintWriter out) throws IOException { System.out.print("Enter your nick: "); String msg = stdIn.readLine(); out.println("NICK " + msg); String serverResponse = in.readLine(); if ("SERVER: OK".equals(serverResponse)) return msg; System.out.println(serverResponse); return getNick(in, out); } public static void main (String[] args) throws IOException { Socket server = null; try { server = new Socket(host, port); } catch (UnknownHostException e) { System.err.println(e); System.exit(1); } stdIn = new BufferedReader(new InputStreamReader(System.in)); /* obtain an output stream to the server... */ PrintWriter out = new PrintWriter(server.getOutputStream(), true); /* ... and an input stream */ BufferedReader in = new BufferedReader(new InputStreamReader( server.getInputStream())); nick = getNick(in, out); /* create a thread to asyncronously read messages from the server */ ServerConn sc = new ServerConn(server); Thread t = new Thread(sc); t.start(); String msg; /* loop reading messages from stdin and sending them to the server */ while ((msg = stdIn.readLine()) != null) { out.println(msg); } } } class ServerConn implements Runnable { private BufferedReader in = null; public ServerConn(Socket server) throws IOException { /* obtain an input stream from the server */ in = new BufferedReader(new InputStreamReader( server.getInputStream())); } public void run() { String msg; try { /* loop reading messages from the server and show them * on stdout */ while ((msg = in.readLine()) != null) { System.out.println(msg); } } catch (IOException e) { System.err.println(e); } } } and here's the ChatServer.java: /* ChatServer.java */ import java.net.ServerSocket; import java.net.Socket; import java.io.IOException; import java.io.BufferedReader; import java.io.InputStreamReader; import java.io.PrintWriter; import java.util.Hashtable; public class ChatServer { private static int port = 5000; /* port to listen on */ public static void main (String[] args) throws IOException { ServerSocket server = null; try { server = new ServerSocket(port); /* start listening on the port */ } catch (IOException e) { System.err.println("Could not listen on port: " + port); System.err.println(e); System.exit(1); } Socket client = null; while(true) { try { client = server.accept(); } catch (IOException e) { System.err.println("Accept failed."); System.err.println(e); System.exit(1); } /* start a new thread to handle this client */ Thread t = new Thread(new ClientConn(client)); t.start(); } } } class ChatServerProtocol { private String nick; private ClientConn conn; /* a hash table from user nicks to the corresponding connections */ private static Hashtable nicks = new Hashtable(); private static final String msg_OK = "OK"; private static final String msg_NICK_IN_USE = "NICK IN USE"; private static final String msg_SPECIFY_NICK = "SPECIFY NICK"; private static final String msg_INVALID = "INVALID COMMAND"; private static final String msg_SEND_FAILED = "FAILED TO SEND"; /** * Adds a nick to the hash table * returns false if the nick is already in the table, true otherwise */ private static boolean add_nick(String nick, ClientConn c) { if (nicks.containsKey(nick)) { return false; } else { nicks.put(nick, c); return true; } } public ChatServerProtocol(ClientConn c) { nick = null; conn = c; } private void log(String msg) { System.err.println(msg); } public boolean isAuthenticated() { return ! (nick == null); } /** * Implements the authentication protocol. * This consists of checking that the message starts with the NICK command * and that the nick following it is not already in use. * returns: * msg_OK if authenticated * msg_NICK_IN_USE if the specified nick is already in use * msg_SPECIFY_NICK if the message does not start with the NICK command */ private String authenticate(String msg) { if(msg.startsWith("NICK")) { String tryNick = msg.substring(5); if(add_nick(tryNick, this.conn)) { log("Nick " + tryNick + " joined."); this.nick = tryNick; return msg_OK; } else { return msg_NICK_IN_USE; } } else { return msg_SPECIFY_NICK; } } /** * Send a message to another user. * @recepient contains the recepient's nick * @msg contains the message to send * return true if the nick is registered in the hash, false otherwise */ private boolean sendMsg(String recipient, String msg) { if (nicks.containsKey(recipient)) { ClientConn c = nicks.get(recipient); c.sendMsg(nick + ": " + msg); return true; } else { return false; } } /** * Process a message coming from the client */ public String process(String msg) { if (!isAuthenticated()) return authenticate(msg); String[] msg_parts = msg.split(" ", 3); String msg_type = msg_parts[0]; if(msg_type.equals("MSG")) { if(msg_parts.length