Up to now, we've assumed that once we know the IP address of a machine, we can pass data to it. Unfortunately, that's not quite true. To explain why, let's look at the hardware in a little more detail.
Every machine in the world capable of being connected to a network is assigned a unique number, which is normally burnt onto a chip when the machine is built. Sometimes, as in the case of a Sun, it may be on the main machine motherboard, while at other times it may be built into a third-party network card. This number is known by a variety of names—the Ethernet address, the hardware address, or the Media Access Control (MAC) address.
An address is a 48-bit number usually written as six colon-separated hex bytes. Each manufacturer ...