System information

Troubleshooting AppleTalk 9-177
The Network Layer
baseband signaling, and runs on shielded twisted-pair media at 230.4 kbps. The physical interface is
EIA/TIA-422 (formerly RS-422), a balanced electrical interface supported by EIA/TIA-449
(formerly RS-449). LocalTalk segments can span up to 300 meters and support a maximum of 32
nodes.
The Network Layer
This section describes AppleTalk network-layer concepts and protocols. It includes discussion of
protocol address assignment, network entities, and AppleTalk protocols that provide OSI reference
model Layer 3 functionality.
Protocol Address Assignment
To ensure minimal network administrator overhead, AppleTalk node addresses are assigned
dynamically. When a Macintosh running AppleTalk starts up, it chooses a protocol (network-layer)
address and checks whether that address is currently in use. If it is not, the new node has successfully
assigned itself an address. If the address is currently in use, the node with the conflicting address
sends a message indicating a problem, and the new node chooses another address and repeats the
process. Figure 9-2 shows the AppleTalk address selection process.
The mechanics of AppleTalk address selection are media dependent. The AppleTalk Address
Resolution Protocol (AARP) is used to associate AppleTalk addresses with particular media
addresses. AARP also associates other protocol addresses with hardware addresses. When either
AppleTalk or any other protocol stack must send a packet to another network node, the protocol
address is passed to AARP. AARP first checks an address cache to see whether the relationship
between the protocol and the hardware address is already known. If it is, that relationship is passed
up to the inquiring protocol stack. If it is not, AARP initiates a broadcast or multicast message
inquiring about the hardware address for the protocol address in question. If the broadcast reaches a
node with the specified protocol address, that node replies with its hardware address. This
information is passed up to the inquiring protocol stack, which uses the hardware address in
communications with that node.