URL url = null;
    String linea = "";
    int respuesta = 0;
    StringBuilder resul = null;
    try {
        url = new URL("http://www.nestel.cl/Login.php?user="+user+"&pass="+pass);
        HttpURLConnection conexion= (HttpsURLConnection)url.openConnection();
        respuesta=conexion.getResponseCode();

        resul=new StringBuilder();

        if (respuesta==HttpURLConnection.HTTP_OK )
        {
            InputStream in=new BufferedInputStream(conexion.getInputStream());
            BufferedReader reader=new BufferedReader(new InputStreamReader(in));

            while((linea=reader.readLine())!=null)
            {
                resul.append(linea);
            }
        }

        }
    catch (Exception e){
        Toast.makeText(getApplicationContext(), "error", Toast.LENGTH_LONG).show();
    }

    return resul.toString();
}

int respuesta = 0;
    try {
        JSONArray json = new JSONArray(response);
        if(json.length()>0)
        {
            respuesta = 1;
        }
    }catch (Exception e){}
    return  respuesta;
}

Thread tr = new Thread(){
        @Override
        public void run() {
            final String resultado = enviardatosGet(mirut.getText().toString(), mipass.getText().toString());
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    int r=obtenerdatosJSON(resultado);
                    if(r>0){
                        Intent i = new Intent(getApplicationContext(),menu.class);

                        startActivity(i);
                    }else{
                        Toast.makeText(getApplicationContext(), "usuario o pass incorrecto", Toast.LENGTH_LONG).show();
                    }
                }
            });
        }
    };
    tr.start();
}

Process: com.example.luisalarcon.alumnos, PID: 24002

              java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
                  at android.os.Handler.<init>(Handler.java:200)
                  at android.os.Handler.<init>(Handler.java:114)
                  at android.widget.Toast$TN.<init>(Toast.java:627)
                  at android.widget.Toast.<init>(Toast.java:131)
                  at android.widget.Toast.makeText(Toast.java:431)
                  at com.example.luisalarcon.alumnos.MainActivity.enviardatosGet(MainActivity.java:67)
                  at com.example.luisalarcon.alumnos.MainActivity$1.run(MainActivity.java:93)