get local IPs from the network

Dear Friends
I have Survey web applications and i need to install it in the local web server in my company
the problem is what correct code to get the real ip for the machines to prevent vote more than one
for every employee
note: the company have domain controller and all user belong to Group Policy 
I try many Code like
Request.Servervariables("REMOTE_ADDR")
HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
no correct result give me 127.0.0.1 or the server IP
please any help me for this case....
 
thanks
 
 
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