In linux, you can change the hardware (MAC) address of your machine.This is how it is done.
# ifconfig eth0 down
# ifconfig eth0 hw ether 00:80:48:BA:d1:20
# ifconfig eth0 up
# ifconfig eth0 |grep HWaddr
Why would I want to change the MAC address of my machine – you ask ?
Consider this scenario. I am connected to the internet using a cable modem. And my ISP dynamically allocates the IP address to my machine.