Machan, If your computer has a Ethernet port (RJ45) there is a mac address which hard coded to chip. Also wi-fi connections too get the same mac address. It has 48 bits and the mac address to a network adaptor is unique. (although it can be changed by using softwares).
But the tracing from mac addresses can only be done in LANs. When you connect to internet, the tracing is done by using the IP address that you get. But in SL all the ISPs provide dynamic public ip to connect to internet.....