Query in NetworkInterface?

Hi,

In my application, have a login page there i am checking user mac address also.

using NetworkInterface() as code given below.

Dim nics As NetworkInterface() = NetworkInterface.GetAllNetworkInterfaces()
   Return nics(0).GetPhysicalAddress.ToString()

My doubt is whether any user can hack the mac address ? its possible ? for security reasons i want to know ..pls reply quick..