First let me explain a few things about MAC addresses.
MAC stands for Media Access Control and in a sense the MAC address is a computer’s true name on a LAN. An Ethernet MAC address is a six byte number, usually expressed as a twelve digit hexadecimal number (Example: 1AB4C234AB1F).

IPs are translated to MAC address by a protocol called ARP (Address Resolution Protocol). Let’s say a computer with and IP of 192.168.1.1 wants to send information to another computer on the LAN that has an IP of 192.168.1.2 . First 192.168.1.1 will send out a broadcast to all stations on the LAN asking who has the IP 192.168.1.2. Then the box that has 192.168.1.2 will respond to 192.168.1.1 with it’s MAC address which is cached in 192.168.1.1’s ARP table for later use.

You can see the ARP table of a box by dropping out to a command prompt and typing “arp –a” in Windows or just “arp” in Linux. ARP can also work the other way by a host on the LAN sending its MAC address to another machine on the LAN for preemptive caching unless the host is configured to not accept un-requested ARP replies.

A person might want to change the MAC address of a NIC for one of many reasons:

To get past MAC address filtering on a router. Valid MAC addresses can be found by sniffing them and then the deviant user could assume the MAC of a valid host. This can cause some network stability problem.
Sniffing other connections on the network.
So as to keep their burned in MAC address out of IDS and security logs, thus keeping deviant behavior from being connected to their hardware.
To pull off a denial of service attack, for instance assuming the MAC of the gateway to a sub net might cause traffic problems.

Linux
To change your MAC address in Linux (and most *nix system) is easy as pie. All it takes is two easy to script commands:

ifconfig eth0 down hw ether 00:00:00:00:00:01

ifconfig eth0 up

These two little commands would set you eth0 interface to use the MAC 00:00:00:00:00:01. Just plug in the NIC you want to set and the MAC address you want to use into the commands above and your done. Changing your MAC address is one of those things that is much easier to do in Linux then under Windows.

Windows: The Hard Way

In XP you can use the regedit to edit the registry from a GUI or the reg command to edit it from the console, I’ll be using regedit. Information on all your NICs can be found the registry key

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Contro l\Class\{4D36E972-E325-11CE-BFC1-08002bE10318}\ .
Under this key you will find a bunch of sub keys labeled as 0000, 00001, 0002 and so forth. We can assume any MAC address we want by finding the key that controls the NIC we want to change, putting in a string value called “NetworkAddress” and setting it to the MAC address we want to use formatted as a twelve digit hex number (example: 000000000001).
To find out which key is which we can search through them for the value “DriverDesc” until we find the one that matches the NIC we wish to alter. After you set “NetworkAddress” to the address you want just restart the NIC by disabling it then enabling it (or in the case of PCMCIA cards, just eject and reinsert). You can confirm the MAC address change by using the “getmac” or “ipconfig /all” commands.

Windows: The Easy Way

Use Smac (
Code:

http://www.klcconsulting.net/smac/

) or Etherchange (
Code:

http://ntsecurity.nu/toolbox/etherchange/

). Smac has a nice GUI and was free but has since gone commercial. Since GUIs are for wimps and I’m cheap I use Arne Vidstrom Etherchange. Etherchange is completely free and easy to use, just walk though the interactive dialog, restart your NIC and your done.

BSD

1) Bring down the interface: "ifconfig xl0 down"

2) Enter new MAC address: "ifconfig xl0 link 00:00:00:AA:AA:AA"

3) Bring up the interface: "ifconfig xl0 up"



Linux

1) Bring down the interface: "ifconfig eth0 down"

2) Enter new MAC address: "ifconfig eth0 hw ether 00:00:00:AA:AA:AA"

3) Bring up the interface: "ifconfig eth0 up"



Windows 2000/XP

Method 1:

This is depending on the type of Network Interface Card (NIC) you have. If you have a card that doesn’t support Clone MAC address, then you have to go to second method.

a) Go to Start->Settings->Control Panel and double click on Network and Dial-up Connections.

b) Right click on the NIC you want to change the MAC address and click on properties.

c) Under "General" tab, click on the "Configure" button

d) Click on "Advanced" tab

e) Under "Property section", you should see an item called "Network Address" or "Locally Administered Address", click on it.

f) On the right side, under "Value", type in the New MAC address you want to assign to your NIC. Usually this value is entered without the "-" between the MAC address numbers.

g) Goto command prompt and type in "ipconfig /all" or "net config rdr" to verify the changes. If the changes are not materialized, then use the second method.

h) If successful, reboot your system.


Method 2:

This should work on all Windows 2000/XP systems

a) Go to Start -> Run, type "regedt32" to start registry editor. Do not use "Regedit".

b) Go to "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Contro l\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}". Double click on it to expand the tree. The subkeys are 4-digit numbers, which represent particular network adapters. You should see it starts with 0000, then 0001, 0002, 0003 and so on.

c) Find the interface you want by searching for the proper "DriverDesc" key.

d) Edit, or add, the string key "NetworkAddress" (has the data type "REG_SZ") to contain the new MAC address.

e) Disable then re-enable the network interface that you changed (or reboot the system).


Method 3:

Use the program Etherchange from
Code:

http://ntsecurity.nu/toolbox/etherchange/

Please, take a reg backup before u go in for regedit.