Network diagrams are a very convenient way to represent a network topology and its architecture. They are widely used by nearly all small or home office (SOHO) and enterprise networks. But this representation often lacks a logical layer for providing a better understanding of how network components interact and in which directions network traffic flows. It is not an easy task to show it in a diagram, so network engineers use a bunch of documentation for that purpose, mostly combining tables, flowcharts, and diagrams.
But as we have a very simple network diagram and a pretty straightforward understanding of how network traffic should flow, we can try to depict it as an additional layer on our network diagram, as shown in the following diagram: