MAC (Media Access Control)

Unfortunately, it's not always as simple as the previous page implied. The IP address identifies the machines at a software level, but on the wire, a different type of addressing is used, so some additional information is also required:

Source IP	192.168.1.1	(A)
Source MAC	01:C0:F2:69:31:21	(A)
Destination IP	192.168.1.2	(B)
Destination MAC	03:A0:B3:27:A2:2E	(B)
Data	Hello B! This is the Data

What's that about MAC addresses? Those are the hardware addresses of the network cards installed in those machines. Any device receiving the packet will only process the packet if it matches their hardware address (or the special broadcast address, which we'll deal with in a minute). This address is assigned by the hardware manufacturer, from the address pool allocated to them by the IEEE. If you have an Intel network card, you'll have an Intel MAC address (maybe 00:02:B3:xx:xx:xx). A 3COM network card will have a 3COM MAC address (maybe 00:04:0B:xx:xx:xx). This is also called the Ethernet address, or the Physical address. As the ethernet MAC address is a very large number (displayed in hexadecimal (base 16) for clarity - B's address above converts to the number 3,988,735,369,774), every card in the world can be (and is) unique.

It is possible to change your MAC address, but there is rarely a need to do so.