The administrator cannot browse the site from the server and recieves an error 401.1 Unauthorized

ISSUE DESCRIPTION
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=> SharePoint site is browsable from the client machine with the host header.
=> When browsing from the server, it prompts for credentials three times and then we get 401.1  
      Unauthorized error.
=> On server, we can browse the SharePoint site with server name

Cause :-  This behavior is by design
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RESOLUTION
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There are two methods to work around this issue, use one of the following methods, as appropriate for your situation.

1. Specify host names (Preferred method if NTLM authentication is desired)

2. Disable the loopback check (less-recommended method)

Please check the Below Url for More detailed Description and the steps to follow for the Resolution .

http://support.microsoft.com/kb/896861