How to find out MAC address of remote user

Hello, that's what I wanna find out: The MAC (that's something like a unique network-card number) address of a remote user. I just know I can find it out by backtracing his IP address. But which function to use? By this MAC address you can identify any user, no matter which IP address he uses and what time is between his visits (and no matter which browser he has used, then). Sessions, cookies etc. aren't enough as they aren't as secure as identifying by MAC is. (I know, you can change this number on certain cards, but this isn't as easy and common as deleting cookies, hmm?) Does someone know how can I find it out? Or has maybe someone an idea how to find out the route the IP packages come? (like "tracert" or "route" in windows)("NeoTrace") I would thank for any suggestion, Mark