A MAC (Media Control Access) address is a unqiue 12 digit 48 byte long number, unqiue to every network adapter ever made. The first 3 sets of numbers, or the first 6 numbers are the ID of the network adapter manufacturer, and the last 3 sets, or the last 6 numbers are the ID that the manufacturer gave to that specific adapter. MAC addresses were made to be fixed, unchangable. Also, many ISPs designate an IP to a MAC address so if say a router is added, then the MAC address would change and the ISP might shut down the account. To find out your MAC address, that is if you have a network adapter, then you can use winipcfg or ipconfig if you have windows, or you can use ifconfig -a in linux or unix systems. I have heard that it is possible to change MAC addresses from the My Network Places interface in Windows 2000, though this is not possible for all MACs and I have not been able to confirm this yet. Also I have heard that the same can be done using the ifconfig utility in linux and unix boxes. And one more thing, is that, at least with the ifconfig utility that the MAC changing is not permanent, and only lasts for the duration of the uptime, so when you have to reboot, it would have to be done all over again, though perhaps somebody will automate it sometime.

In using any of the ways described below you absolve me of any responsibility or wrongdoing for any harm done to your computer.

To change the address in linux on ethernet card eth0, try using something like this,
#ifconfig eth0 hw ether XX:XX:XX:XX:XX:XX

In Windows2k try going to your "control panel", then double clicking your "network and dial up connections", right click the connection that uses the NIC (network adapter) and click properties. Then press "configure" and then "advanced" and then "locally administered address". Then enter the new MAC address.
This can also be done in the registry, using Regedt32, go to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<Your NIC>
And enter your new MAC address. You must reboot for the registry changes to take effect, after you have done so, try typing "net config rdr" in the command prompt to see if the changes have taken effect or not.