This is an open question about the domains for SharePoint apps. On Technet: Configure an environment for apps for SharePoint (SharePoint 2013) we can read the following:

You must configure a new name in Domain Name Services (DNS) to host the apps. To help improve security, the domain name should not be a subdomain of the domain that hosts the SharePoint sites. For example, if the SharePoint sites are at Contoso.com, consider ContosoApps.com instead of App.Contoso.com as the domain name.

As a SharePoint architect or a SharePoint developer, you must have been thinking about the benefits/limitations of SharePoint apps a lot. I want to point out one of them today, which is very important: using custom webservices deployed to SharePoint inside apps. That is impossible and it is designed to be so due to the security architecture in the sharepoint app framework. I have read much about SharePoint apps (books, whitepapers, blog posts) and stumbled over these two contradictive statements: