The AppleTalk packet is encapsulated within an Ethernet packet. The
Ethernet packet contains the Ethernet address of the receiving device, so
only that device will accept and inspect the packet. Any AppleTalk device
connected directly to an Ethernet uses Address Resolution Protocol (ARP) to
determine the Ethernet address of the AppleTalk device it wants to send a
packet to.
This means that all address resolution and Ethernet packet addressing occur
within the sending AppleTalk node. For example, a Macintosh II, using an
EtherTalk card and the EtherTalk drivers and sending to another Macintosh
II on the same Ethernet, inspects its ARP address tables and sends an
AppleTalk packet encapsulated within an Ethernet packet addressed
specifically for the receiving Macintosh II. The receiving Macintosh II
strips the Ethernet information from the packet and processes the AppleTalk
packet. No other Ethernet device does anything with that packet, unless it
is a device like a protocol analyzer peeking at packets.
In the case of an AppleTalk device on a LocalTalk on the other side of a
bridge (like a Kinetics FastPath), the Ethernet packet is addressed to the
FastPath. The FastPath takes the packet, strips off the Ethernet
information, and sends the packet out onto the LocalTalk network.