The network layer

Entities on the network layer manage interactions over the network topology. They're responsible for address resolution and routing data to target hosts once an address has been resolved. They also handle message delivery based on constraints or the resource availability of the physical network. So, while the transport layer determines the interactions between host-level tiers of the network stack on either side of a connection, the network layer is responsible for applying the transport protocol across the chain of devices that form the route between two hosts. The distinction between adjacent layers can be subtle, and we'll discuss some of the responsibilities specific to the network layer later in this chapter. So, if the distinction between transport and network layers is unclear, trust that we'll (at least, attempt to) clarify that distinction later.