ALAP is also responsible for assigning node numbers to each station on a network. Carrier sense means that a sending node first listens to the network to hear if any other node is using the bus and defers to the ongoing transmission. Collision avoidance means that the protocol attempts to minimize collisions between transmitted data packets. An internet can be formed, for example, by using a bridge between two, or more, AppleTalk networks. A socket is a communication endpoint within a node on the network.
Sockets belong to processes or functions that are implemented within software in the node.
One Mac may have several AppleTalk connections open at one time, so the node number is not enough to identify a network address. In addition, node numbers are unique only within a single physical network, so DDP requires that each network be assigned a network number. The Datagram Delivery Protocol takes care of assigning socket numbers, as well as node numbers and network numbers, to provide a unique identification for every process occurring on the AppleTalk network. As we move on to the Transport Layer, several protocols exist to add different types of functionality to the underlying services. The routing tables pair network numbers with the local node number of the bridge through which the shortest path to that net exists.