The first two layers of the OSI Reference Model, the physical layer and data link layer, deal primarily with physical network details. The various LAN, WLAN and WAN protocols function primarily at these two layers to connect devices to create networks, and perform functions such as physical connection and signaling, media access control and local delivery of data between devices on the same network. Above these layers, we move beyond the hardware aspects of networking and closer to the more abstract realm of software-related network functions.

The third OSI layer is the network layer. We are of course talking about networks in this Guide, and it is no coincidence that the layer bearing that name is one of the most important in comprehending how networks function. It is here that we find protocols that tie networks together to create internetworks, and also where cross-network addressing and routing are performed. The network layer is also called the internet layer in the TCP/IP model.

In this section I provide details for the various TCP/IP protocols that reside architecturally at the TCP/IP internet layer / OSI network layer. Much of the focus here is on the all-important Internet Protocol; the section covering IP includes extensive coverage of IP version 4, IP version 6, and IP-related protocols such as IPSec, Mobile IP and IP Network Address Translation (NAT). The following three subsections cover IP support protocols such as the Internet Control Message Protocol (ICMP) and IPv6 Neighbor Discovery (ND) protocol, and the complete set of IP routing protocols.