Physical layers clearly map to physical topology, and data link doesn't stray far. In the network layer, Layer 3 of OSI and the Internetwork layer of the Internet model, we now deal with logical topologies of multiple media, not just individual media. Once the network layer learns the logical path, it forwards along it, unless Murphy gets involved.

Certain network layer devices (e.g., Network Address Translators), break Layer 3 architectural principles in a controlled way as long as Murphy is kept away. Some of these controlled violations involve limited interaction with the transport layer, which I will introduce here to the extent that it interacts with network. Most transport issues will be discussed in the next article on the layers above network.

Network layer

The network layer involves learning about paths that may go through multiple media. The ways of the network layer involve the transition from one medium to the next, until the final medium on the path is reached, and it can have multiple active links between pairs of points. Network contributes to the end-to-end or transport subsystem, much as the longest journey begins with a single step.

Classical network layer assumes: