Take a look at this article on MSDN.
http://msdn.microsoft.com/en-us/library/hh556233.aspx[
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Specifically the section that states the following:
In order for Windows Store applications to make cross machine calls you must enable another capability called “Home/Work Networking”. This setting is also in the Package.appmanifest file under the Capabilities tab. Select the Home/Work Networking checkbox. This gives your application inbound and outbound access to the networks of the user’s trusted places like home and work. Inbound critical ports are always blocked. For accessing services on the internet you must also enable Internet (Client) capability.