The network interface layer defines the protocols and hardware required to deliver data across some physical network. The term network interface refers to the fact that this layer defines how to connect the host computer, which is not part of the network, to the network; it is the interface between the computer and the network. For instance, Ethernet is one example protocol at the TCP/IP network interface layer. Ethernet defines the required cabling, addressing, and protocols used to create an Ethernet LAN. Likewise, the connectors, cables, voltage levels, and protocols used to deliver data across WAN links are defined in a variety of other protocols that also fall into the network interface layer.
Chapter 3, “Data Link Layer Fundamentals: Ethernet LANs,” and Chapter 4, “Fundamentals of WANs,” cover more details about the TCP/IP network interface layer.
Just like every layer in any networking model, the TCP/IP network interface layer provides services to the layer above it in the model. The best way to understand the basics of the TCP/IP network interface layer is to examine the services that it provides to IP.
IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology, things such as which routers are connected to each other, which host computers are connected to which networks, and what the IP addressing scheme looks like. However, the IP protocol purposefully does not include the details about each of the underlying physical networks. Therefore, the Internet layer, as implemented by IP, uses the services of the network interface layer to deliver the packets over each physical network, respectively.
