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552 C
HAPTER
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PPLE
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OUTING
AppleTalk Address
Resolution Protocol
(AARP) Cache
The AppleTalk Address Resolution Protocol (AARP) maps the hardware
address of an AppleTalk node to an AppleTalk protocol address. AARP
maps addresses for both extended and nonextended networks.
AppleTalk uses dynamically assigned 24-bit addresses that consist of a
16-bit network number and a unique 8-bit node number. AppleTalk
networks support a hierarchal addressing scheme in the form of a
network range, with each 16-bit network number within that range
capable of supporting up to 254 nodes.
All AppleTalk nodes, including router interfaces, dynamically acquire a
unique AppleTalk address using a feature provided by the AppleTalk
Address Resolution protocol, called Probe.
When a node on the network initializes, it randomly selects an AppleTalk
address for itself. At the same time, the node sends 10 AARP probe
packets. The probe packets determine whether any other nodes on the
network are using the selected address. If the address already exists, the
initializing node randomly selects another address and sends another set
of probe packets.
AARP maintains an Address Mapping Table (AMT) with the most recently
used hardware addresses and their corresponding AARP addresses. If an
address is not in this table, the router broadcasts AARP requests to all
other AppleTalk nodes on the link to determine the MAC address
mapping for the specified AARP address. The router then creates a
corresponding AMT entry to reflect the new mapping when the
destination node replies. You can view this table, called the
AARP cache ,
through the Administration Console.