Android : Android get ConnectTimeoutException for JsonPost

My code often with both of the following error. My code, server are ok. after ping server i can get correct ping and nslookup can return server detailes but in Android i get ConnectTimeoutException error and i dont khnow whats problem.

public class JsonService {
    private JSONObject json = new JSONObject();

    public JsonService(final String username, final String password, final String functionName) throws Exception {
        json.put("username",     username);
        json.put("password",     password);
        json.put("function_name", functionName);
    }

    public String request() throws ExecutionException, InterruptedException {
        parseJson foo = new parseJson();
        FutureTask<String> futureTask = new FutureTask<String>(foo);
        new Thread(futureTask).start();
        return futureTask.get();
    }

    private String convertToString(InputStream inputStream) {
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
        StringBuilder builder = new StringBuilder();
        try {
            String line;
            while ((line = reader.readLine()) != null) {
                builder.append(line);
            }
            return builder.toString();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return "";
    }
    public class parseJson implements Callable<String> {
        @Override
        public String call() {
            String result = null;
            HttpClient client = new DefaultHttpClient();
            HttpConnectionParams.setConnectionTimeout(client.getParams(), 50000);
            HttpResponse response;
            try {
                HttpPost post = new HttpPost(G.SERVER_REQUEST);
                StringEntity se = new StringEntity(json.toString());
                se.setContentType(new BasicHeader(HTTP.CONTENT_TYPE, "application/json"));
                post.setEntity(se);
                response = client.execute(post);
                StatusLine statusLine = response.getStatusLine();
                if (response != null) {
                    InputStream stream = response.getEntity().getContent();
                    result = convertToString(stream);
                }
            } catch (Exception e) {
                e.printStackTrace();
                Log.e("Error", String.valueOf(e));
            }
            return result;
        }
    }

}