How to detect new Machine like Facebook?

Question

I am actually trying to detect a person, using a computer, Just like facebook. Whenever you sign into facebook using different machine, it detects the new machine and asks you t

Answer 1

I don't know what does facebook do but I suppose they check a cookie.
To be sure, you can setup some HTTP proxy like Fiddler and watch all HTTP headers being sent.

No, MAC address isn't accessible over internet and IP is not the same thing as "Machine"

Answer 2

I checked my facebook cookies and (contrary to @Mattis) i found a cookie called c_user, i think that is the actual cookie that holds your id in a 15 digit decimal number, also another persistent cookie called datr that stores a 21 digit string with _ at the end

I hope i helped