AppleTalk by Apple uses addresses that aren’t much longer than DECnet addresses: 24 bits, with 16 bits for the network and 8 bits for hosts. As with IPX, the network address is learned from routers, but unlike IPX and DECnet Phase IV, AppleTalk doesn’t use fixed information such as the Ethernet MAC address or a manually configured value to arrive at a host address. Instead, a host simply picks an address and checks if it’s already in use by sending a message to this address. If there is no answer, the address is free, and the host may start using it. If there is an answer, the address is already in use, so the new host picks another address and retries. AppleTalk uses an address resolution protocol similar to that of IP (discussed later this chapter) to find Ethernet MAC addresses for other AppleTalk hosts it only knows the AppleTalk address for. Figure 1-4 shows how the AppleTalk address is created and how it’s resolved into an Ethernet MAC address.
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