On the server inside the webservice method call, you have access to the HttpContext object. Using HttpContext.Current, you have access to a number of objects (including the request object).
From the request object you can find out information, including the host address from which the request comes from.
If you need to map this address to a hostname , you can use UserHostName property instead
' Example web method
<WebMethod()> _
Public Function TestMessage( ) as String
Trace.WriteLine("GetMessage called from : " + HttpContext.Current.Request.UserHostAddress + " " + HttpContext.Current.Request.UserHostName )
Return "Hello"
End Function