How do I find a client machine individually using asp.net ?

Question

Hi,
My Name is Raja A,
In my asp.net web application i need to find every client using my application uniquely
by using any unique identification of client machine.

I tried to find the 'MAC' address of client by the use of "Web Server Control"
but while i do so i'm getting the same server "MAC" for all the client machines.

Can anyone help me out, with this problem.
(Think i'm a beginner please) .