Example Scenario: You want to add another user-friendly URL (http://inside.contoso.com) to an existing SharePoint 2010 or 2013 web application in addition to the default URL (http://servername:5555). Then users can access the web application using either the default or user-friendly URL.
To be in a Microsoft supported state, every alternate access mapping URL in SharePoint must have a backing IIS site which means that the web application has to be extended into another zone. What this means is you cannot have multiple public URLs in same AAM zone for one SharePoint web application. Multiple internal URLs can be associated with a single public URL. But each zone can have only a single public URL. If alternate access mappings are configured manually without extending web application, unexpected authentication failures could occur on some sites depending on authentication settings. http://support.microsoft.com/kb/2624320
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