Read original post by Diederik Krols at U2U Blog
This article shows how a side-loaded line-of-business Windows app can communicate through HTTP with a locally installed desktop component (a .NET class library) by making use of the Open Web Interface for .NET. Apps that are loaded from the Store can still not leave the sandbox and can only connect to servers, but since Windows 8.1 side-loaded Enterprise apps are allowed to open ports in the firewall around their own app container. Such apps are now allowed e.g. to connect to http://localhost. For more info, just check the first half of this Build 2014 session.