Physical addresses contain two parts. The first part, known as theOUI (OrganizationallyUnique Identifier), is a character sequence assigned by IEEE that identifies the NIC’s manufacturer. For example, a series of Ethernet NICs manufactured by the 3Com Corporation begins with the hexadecimal characters“00608C,”while a series of Ethernet NICs manufactured by Intel begins with“00AA00.”Some manufacturers have several different OUIs. IEEEalso uses the termcompany_idto refer to the OUI. Traditionally, this portion of a physicaladdress is sometimes called theblock ID.The remaining characters in a physical address, known as theextension identifier, identifythe interface. Vendors such as 3Com and Intel assign each NIC a unique extension identifier,based on the NIC’s model and manufacture date. By assigning unique extension identifiers,companies ensure that no two NICs share the same physical address. Extension identifiersmay also be known asdevice IDs.In traditional physical addressing schemes, the OUI is six characters (or 24 bits) long and theextension identifier is also six characters long. Together, the OUI and extension identifierform a whole physical address. For example, IBM might assign one of its NICs the extensionidentifier 005499. The combination of the IBM OUI and this extension identifier result in aunique, 12-character, or 48-bit address of 00608C005499.Physical addresses are frequently depicted as hexadecimal numbers separated by colons—forexample, 00:60:8C:00:54:99.Whereas the traditional MAC addressing scheme assigns interfaces a 48-bit address, IEEE’s newerEUI-64 (Extended Unique Identifier-64)standard callsfor a 64-bit physical address. In the EUI-64 standard, the OUI portion is 24 bits in length. A40-bit extension identifier makes up the rest of the physical address to total 64 bits.If you know a computer’s physical address, you can determine which company manufacturedits NIC by looking up its block ID. IEEE maintains a database of block IDs and their manufacturers, which is accessible via the Web. At the time of this writing, the database searchpage could be found athttp://standards.ieee.org/regauth/oui/index.shtml.Figure 2-10 provides a simple view of a frame created at the Data Link layer. Notice thefields reserved for the physical, or MAC, addresses belonging to the transmission’s destination and source nodes. The purpose of the preamble field in this frame is to synchronize signaling between nodes. You will learn more about Ethernet frames in Chapter 5.Because of their hardware addressing function, NICs can be said to perform in the Data Linklayer of the OSI model. However, they also perform services in the Physical layer, which isdescribed next.Physical LayerThePhysical layeris the lowest, or first, layer of the OSI model. Protocols at the Physicallayer accept frames from the Data Link layer and generate signals as changes in voltage atthe NIC. (Signals are made of electrical impulses that, when issued in a certain pattern, represent information.) When the network uses copper as its transmission medium, these signalsare also issued over the wire as voltage. In the case of fiber-optic cable, signals are issued aslight pulses. When a network uses wireless transmission, the signals are sent from antennasas electromagnetic waves.When receiving data, Physical layer protocols detect and accept signals, which they pass onto the Data Link layer. Physical layer protocols also set the data transmission rate and monitor data error rates. However, even if they recognize an error, they cannot perform error correction. When you install a NIC in your desktop PC and connect it to a cable, you are establishing the foundation that allows the computer to be networked. In other words, you areproviding a Physical layer.Simple connectivity devices such as hubs and repeaters operate at the Physical layer. NICsoperate at both the Physical layer and at the Data Link layer. As you would expect, physicalnetwork problems, such as a severed wire or a broken connectivity device, affect the Physicallayer. Similarly, if you insert a NIC but fail to seat it deeply enough in the computer’s maincircuit board, your computer will experience network problems at the Physical layer.Most of the functions that network administrators are most concerned with happen in thefirst four layers of the OSI model: Physical, Data Link, Network, and Transport. Therefore,
the bulk of material in this book and on the Network+ exam relates to these four layers.
Software programmers, on the other hand, are more apt to be concerned with what happens
at the Application, Presentation, and Session layers.
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