It depends on what you mean by 'mobile application'. There are several mobile enabled options for CRM 2013, some of them outlined in this blog post.
http://community.dynamics.com/crm/b/forceworksblog/archive/2013/10/10/dynamics-crm-2013-feature-dive-mobile
If you are trying to use the iOS/Android/Windows Mobile app, your server will need to be internet facing, or you will only be able to access it if the mobile device is also on the same internal network as CRM.
Hope this helps!