By default, when Microsoft Internet Security and Acceleration (ISA) Server 2006 receives an incoming Web request, it determines whether the request is allowed, and then routes the request to the appropriate location on the Web server as defined in the Web publishing rule. ISA Server does not pass the host header (for example, Host: example.microsoft.com) included in the client request to the published server. Instead, ISA Server substitutes the original host header in the request with the name of the server specified in the Web publishing rule. As a result, all requests that are routed to a particular Web server are sent to the same (default) Web site on that Web server.
You can configure ISA Server to pass the original host header information, thereby allowing client requests to be routed to a particular site on the Web server. For example, when you are publishing more than one Web site on the same Web server, the original host header is required for the request to be routed to the intended Web site.
For instructions, see Configure name of Web server to publish.