Alternate access mappings enable multiple internal URLs to be mapped to a single public URL. An internal URL is the URL of a Web request as it is received by Office SharePoint Server 2007 or Windows SharePoint Services 3.0. A public URL is the URL of an externally accessible Web site.
The public URL is the base URL that Office SharePoint Server 2007 and Windows SharePoint Services 3.0 use in the pages that they return in response to Web requests. Alternate access mappings support Internet deployment scenarios in which the URL of a Web request is not the same as the URL that was typed by an end user because the URL has been modified by a reverse proxy.
A reverse proxy sits between end users and Web servers. Requests to a Web server are first received by the reverse proxy and, if those requests pass the proxy's security filtering, the proxy forwards the requests to the Web server. Reverse proxies can be configured to receive a Web request over the Internet by using HTTPS (Hypertext Transfer Protocol over Secure Socket Layer), and then forward the request to a Web server by using HTTP. This is referred to as off-box SSL termination.
Alternate access mapping collections can contain up to five authentication zones, but each zone can only have a single public URL. Mapping collections correspond to the following authentication zones:
Administrators need to make sure that alternate access mappings are configured correctly for every SharePoint deployment, no matter how simple or complex. For more information about alternate access mappings, see:
Plan alternate access mappings (Windows SharePoint Services)
Plan alternate access mappings (Office SharePoint Server)
For more information about authentication for Windows SharePoint Services 3.0 and Office SharePoint Server 2007, see the Authentication Resource Center for SharePoint Products and Technologies.
Douglas Goodwin, Writer SharePoint Server UA team