Data Link Addresses

In a certain community in Colorado, two individuals are named Jeff Doyle. One Jeff Doyle frequently receives telephone calls for the person with whom he shares a name—so much so that his clever wife has posted the correct number next to the phone to redirect errant callers to their desired destination. In other words, because two individuals cannot be uniquely identified, data is occasionally delivered incorrectly and a process must be implemented to correct the error.

Among family, friends, and associates, a given name is usually sufficient for accurately distinguishing individuals. However, as this example shows, most names become inaccurate over a larger population. A more unique identifier, such as a United States Social Security number, is needed to distinguish one person from every other.

NOTE

Frame

Devices on a LAN must also be uniquely and individually identified or they, like humans sharing the same name, will receive data not intended for them. When data is to be delivered on aLAN , it is encapsulated within an entity called a frame, a kind of binary envelope. Think of data encapsulation as being the digital equivalent of placing a letter inside an envelope, as in Figure 1.1[1] . A destination address and a return (source) address are written on the outside of the envelope. Without a destination address, the postal service would have no idea where to deliver the letter. Likewise, when a frame is placed on a data link, all devices attached to the link "see" the frame; therefore, some mechanism must indicate which device should pick up the frame and read the enclosed data.

[1] As will be seen later, creating a data link layer frame is really more like putting an envelope inside a larger envelope.

Figure 1.1. Encapsulation means putting data into a frame—a kind of digital "envelope" for delivery.

Figure 1.1. Encapsulation means putting data into a frame—a kind of digital "envelope" for delivery.

Figure 1.2 shows the format of most common LAN frames. Notice that every case includes a destination address and a source address. The format of the address depends on the particular MAC protocol, but all the addresses serve the same purpose: to uniquely identify the machine for which the frame is destined and the device from which it was sent.

Figure 1.2. The frame format of a few common LAN data link frames.

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The three most common data links currently used in LANs are Ethernet, Token Ring, and FDDI. Although each link is drastically different from the others, they share a common format for addressing devices on the network. This format, originally standardized by Xerox's Palo Alto Research Center (PARC)[2] and now administered by the Institute of Electrical and Electronics Engineers (IEEE), is variously called the burned-in address,[3] the physical address, the machine address, or most commonly, the MAC address.

[2] The full name, as reading any modern text on networking will tell you, is The Now Famous Xerox PARC.

[3] The address is usually permanently programmed, or burned in, to a ROM on the network interface.

The MAC address is a 48-bit number, which, as Figure 1.3 shows, is designed so that every device anywhere on the planet should be uniquely identifiable. Most everyone has heard the legends of large batches of network interface cards being turned out with identical burned-in addresses by unscrupulous "cloning" companies or as the result of "stuck" programming code. Although most of those stories are nothing more than legends, one can imagine what would happen if all devices on a LAN had the same MAC address: Imagine a town in which every resident is named Wessvick Smackley. Men, women, children, dogs, and cats all named Wessvick Smackley. Everyday communication, not to mention the career of the town gossip, would be unimaginably difficult.141

[4] In real life, duplicate MAC addresses on a network are most likely to occur as the result of network administrators using locally administered addresses. This occurrence is common enough on Token Ring networks that one step of the Token Ring insertion process is a duplicate address check.

Figure 1.3. A MAC address.

Figure 1.3. A MAC address.

Although the MAC addresses are by convention referred to as "addresses," they are really names. Think about it: Because the identifier is burned in, or permanently assigned, to a device, it is a part of that device and goes wherever the device goes.[51

[5] Although some data link addresses may be or must be administratively configured, the point is that they are identifiers, unique within a network.

Most adults have several street addresses through their lives, but few have more than one given name. A name identifies an entity—whether a person or a PC. An address describes where that person or PC is located.

In the interest of clarity, this book uses the term data link identifier or MAC identifier instead of MAC address. The reason for making such a distinction will soon be clear.

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