Apple IP (AppleShare IP) Gateway - Manual (Page 11)

How the gateway works
On both AppleTalk and IP networks, data is broken down into packets for
transmission. The two network types use different sets of rules—protocols—
for packet construction and for addressing packets to their correct destinations.
Data can’t ordinarily cross from one network type to the other. The Apple IP
Gateway solves this problem. Installed on a Macintosh computer that is
connected to both an IP network and an AppleTalk network, the Apple IP
Gateway makes addressing transparent on both sides, so communications can
pass freely.
IMPORTANT The gateway computer must be connected to IP by Ethernet cabling.
Client computers may be networked by LocalTalk, Ethernet, or other AppleTalk-
compatible cabling types. Both the client computers and the gateway computer
must have MacTCP software installed. This allows them to “talk” IP, even
though they are using AppleTalk as the underlying network protocol.
Users who want to access an IP service construct their requests in the
appropriate IP format. The packets have all the information necessary for
transport and reassembly on the IP side. Speaking metaphorically, the packets
are enclosed in IP “envelopes.” To get this message to the gateway, the IP
packets are encapsulated in Datagram Delivery Protocol (DDP) packets; DDP
is the AppleTalk standard for data transport. The IP envelope, in other words,
is put inside a DDP envelope for shipment to the gateway. On arrival, the
gateway strips off the DDP envelope and sends the IP packet on its way.
When the IP host sends back a reply, the process is reversed. The gateway
encapsulates the IP data in a DDP envelope and transmits it to the client
over AppleTalk.
Figure 1 shows how the gateway makes it possible to move data from an
AppleTalk network to an IP network.
How the gateway works 3