Most network hardware, be it in a computer, a PDA or a component, has a Media Access Control (MAC) address associated with it. This is a unique number, often written in the format 00-00-00-00-00-00, that is permanently assigned to one exact piece of network equipment. From it you can work out such things as what company made it and so on.

More critically it is often used as at least one part of network security with regard to wireless networks. This, combined with the (usually WEP) encryption key that the other part of "bog-standard" wifi security, does indeed sound like a very good idea insomuch as someone wanting to access that particular network would need a specific piece of hardware (identified by its MAC address) and a specific piece of knowledge (the encryption key). Indeed this is the upper limit of security that many older or cheaper network set ups have available to them.

ifconfig /allThe Poorhouse set out to see how one could go about faking the MAC address. As it is burnt in to each bit of equipment concerned, it was thought it might be at least a touch technical, possibly involving some Linux action, dodgy drivers or grey-market software. Apparently not. Windows XP for instance appears to have exactly this feature built in.

Firstly, to check your MAC address, open up a command prompt, and type ipconfig /all. In the resulting screen of information, look for an entry such as "Physical address". The following line of hex-digits is your MAC address.

Network adaptor driver propertiesIf you're lucky, you get to do it the near-trivially easy way. In summary, to change it you need to access the driver properties of the network card. Go to the Network Connections part of Windows XP's Control Panel. Right click the relevant network connection and choose Properties. Making sure the right network adaptor is selected in the resulting window, select the Configure button. Choose the Advanced tab. There is then a list of various properties you can change. Most often the MAC address option will be called "Network Address", "Ethernet Address" or "Locally Administered Address". If present, simply select it and fill it in with your desired address. Job done. Or if it isn't, reboot and it should be. Anything operating on MAC-based security will likely think you are a different computer. Hack da planet. To put the address back, just undo the change you made.

If this option isn't present, then if you're a brave boy or girl you can go mess with the registry at your very much own risk.

Run the (built-in) file "regedt32" in order to open the registry editor. Now locate the following key:

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

The network adaptor's registry entryIt's an inconveniently long name, but there is only one of it so you can find it with a modicum of effort. Under that key there will be several subkeys that are 4 digits long. Simply flick through them all until you find the one whose "DriverDesc" contains the name of your network card. Once you have found it, see if there is an entry for "NetworkAddress". If there isn't, make one by going to the Edit menu, select New, then String Value. Either way, once you have found the key, simply type the MAC address you want as its value. Just put a string of the 12 numbers/letters in, without separating it with colons, spaces, dashes or any other item. Reboot your computer, and you're done (assuming you didn't break your computer). Again, to put it back, just undo whatever you did to make it happen.

For ultimate fear or laziness there are indeed also programs out there to do the job for you, if you trust them more than you trust yourself. On Windows, these include SMAC, Mac Makeup, Macshift and no doubt many more.

People who've forgotten the MAC codes that are allowed to access their network (or evil hax0rs wanting to access someone else's) might be thinking that's all well and good, but how would you know what MAC address you need to assign your computer to in order to get on the network? There are a few answers.

Firstly, if you can get on a computer that does work on the network, find its MAC address, for instance by using the ipconfig /all command mentioned above. It also is quite possible that the MAC address is physically written somewhere on the device, especially if it is something portable like a PDA or a PCMCIA network adaptor.

Alternatively, as long as there are computers that are using the network, the MAC addresses are transmitted in plaintext as the computers talk to each other and the router. They are therefore particularly easy to get hold of on a wifi network. Get yourself a "packet sniffer" type program that analyses the content of traffic on a network, and most likely that will tell you any active client's addresses with ease. For instance, using Kismet under Linux, highlight the network you are interested in "sniffing", and press Shift and C together. This will bring you up a list of clients currently using the access point, along with their MAC addresses. Ethereal for Windows and Linux will almost certainly do the same thing. As soon as one of the working clients disconnects, you can impersonate them and connect to their network using their MAC address.

Be careful what you discover in these magic packets mind: some poor guy discovered his girlfriend of 17 years was having an affair by sniffing her packets. Well, "packets" is the official story anyway.