SharePoint 2010 Alternate Access Mappings Always Resolve To Port 80

In SharePoint 2010 environment I have some of my web applications running on different ports e.g. http://my-sharepoint-server:12345

In the AAM settings in Central Administration I have the default zone AAM set to http://my-sharepoint-server:12345 and the intranet zone set to http://myapp.mydomain.net

But the myapp.mydomain.net always resolves to port 80 and not to port 12345.

How do I make the AAM URL resolve to port 12345?