Читать книгу Understanding Infrastructure Edge Computing - Alex Marcham - Страница 46

Referred to as the network layer, layer 3 is the only layer of the OSI model which is responsible for internetwork addressing, where network endpoints can be assigned globally unique addresses. This is crucial as without this function, endpoints that are located in other networks would not be reachable and the internet (as a network of networks) would not be able to exist as it does today.

The globally unique addresses which layer 3 provides are mapped on to the link local addresses that are used by protocols operating at layer 2 by a process that is specific to each of those lower layer protocols. This ensures that when data sent from an endpoint on an external network targeted for an endpoint on the local network enters the local network, it can be correctly forwarded using its link local address (such as a MAC address in the case of Ethernet) as if that traffic had originated on the local network to begin with. The external network endpoint is unaware of this process entirely.