No such host is known

Hi All,
Was hoping someone could help with this.  Have configured a web app to read the machine of the remote computer usign the following code. 

       // Get the REMOTE computer name.
        string s1 = Request.ServerVariables["REMOTE_HOST"];
        string hostNameLocal = Dns.GetHostEntry(s1).HostName;
 
I should poijt out that this is going to be an intranet applciation.  Im receiving the following server error.
Was hoping someone had come across this before. Tried searching but couldnt find anything helpful. 
Many Thanks - DJ  

Server Error in '/Final_Sol' Application.
--------------------------------------------------------------------------------

No such host is known
Description: An unhandled exception occurred during ...