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Addressing at the Data Link Layer
The Data Link layer maintains device addresses that enable messages to be sent to a particular device. The addresses are called physical device addresses. Physical device addresses are unique addresses associated with the networking hardware in the computer. In most cases (for example, Ethernet and Token Ring), the physical device address is burned into the network interface card at the time the card is manufactured.

The standards that apply to a particular network determine the format of the address. Because the address format is associated with the media access control method used, physical device addresses are frequently referred to as MAC addresses.

The device address is not actually used to route a message to a specific device. Frames on LANs are typically transmitted so that they are available to all devices on the network. Each device reads each frame far enough to determine the device address to which the frame is addressed. If the frame’s destination address matches the device’s own physical address, the rest of the frame is received. If the addresses do not match, the remainder of the frame is ignored.

As you learn in this chapter, bridges can be used to divide large networks into several smaller ones. Bridges use physical device addresses to determine which frames to leave on the current network segment and which to forward to devices on other network segments. (Chapter 6 covers bridges in more detail.)

Because they use physical device addresses to manage frame routing, bridges function at the level of the Data Link layer and are Data Link layer connectivity devices.