Change MAC address of you Network Adaptor

Although physical MAC (Media Access Control) addresses are permanent by design,

several mechanisms allow modification, or "spoofing", of the MAC address that is

reported by the operating system. This can be useful for privacy reasons, for instance

when connecting to a Wi-Fi hotspot, or to ensure interoperability. Some internet

service providers bind their service to a specific MAC address; if the user then

changes their network card or intends to install a router, the service won't work

anymore. Changing the MAC address of the new interface will solve the problem.

Similarly, some software licenses are bound to a specific MAC address. Changing

the MAC address in this way is not permanent: after a reboot, it will revert to the

MAC address physically stored in the card. A MAC address is 48 bits in length.
As a MAC address can be changed, it can be unwise to rely on this as a single method

of authentication. IEEE 802.1x is an emerging standard better suited to authenticating

devices at a low level.

Mac OS X
Under Mac OS X, the MAC address can be altered in a fashion similar to the

Linux and FreeBSD methods:
sudo ifconfig en0 lladdr 00:01:02:03:04:05
or
sudo ifconfig en0 ether 00:01:02:03:04:05
This must be done as the superuser and only works for the computer's ethernet card.

Instructions on spoofing AirPort Extreme (2.0) cards are available here. There are not,

as of yet, any known ways to spoof original AirPort (1.0) cards.
The AirPort Extreme MAC address can also be changed easily with SpoofMac.
Windows
Under Windows XP, the MAC address can be changed in the Ethernet adapter's

Properties menu, in the Advanced tab, as "MAC Address", "Locally Administered

Address", "Ethernet Address" or "Network Address". The exact name depends on the Ethernet driver used; not all drivers support changing the MAC address in this way.
However, a better solution - requiring Administrative User Rights - is to pass over

the System Registry Keys under

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}.

Here settings for each network interface can be found. The contents of the

string value called 'NetworkAddress' will be used to set the MAC address

of the adapter when next it is enabled. Resetting the adapter can be accomplished

in script with the freely available command line utility devcon from Microsoft,

or from the adapters context menu in the Network Connections control panel applet.
There is a nice tool to change the MAC address for all cards (even those that can't

be changed through the adapter's Properties menu): SMAC MAC Address Changer
Note: to check your MAC address easily on a Windows XP box, go to Run, type CMD,

then type "ipconfig /all" without quotation in the command prompt. The number under

physical address is the MAC address. If multiple IP are displayed, you should look

under the label "Ethernet adapter x", where x is the name of your connection

(which is Local Area Connection by default).

Router
The method to change the MAC address of a router varies with the router.

Not all routers have the ability to change their MAC address. The feature is

often referred to as "clone MAC address". This take the MAC address of one

of the machine on your network and replaces the router's existing MAC address

with it. Some support the option to manually enter the MAC address.