Getting the user login id in a web based application

I am working on an company internal web based application and the application needs the user identity.

in web config I have

identityimpersonate="true"

in vb code I have used

System.Security.Principal.WindowsIdentity.GetCurrent()

WindowsIdentity.Name

or

HttpContext.Current.User.Identity

On the local machine they both return the user login id, but when running the app from a client machine it returns blank. Any idea what could be wrong. I appriciate your help