Laravel subdomain routing is not working

I'm trying to have an admin subdomain (like this)

Route::group(['domain' => 'admin.localhost'], function () {
    Route::get('/', function () {
        return view('welcome');
    });
});

but admin.localhost acts just like localhost. How I'm supposed to do this correctly?

I'm using Laravel 5.1 and MAMP on OSX

via Chebli Mohamed