The reference models demonstrated a standard way for data to communicate between two nodes. Of course, by now, we all know that not all nodes are created equal. Even in its DARPA-net days, there were workstation nodes, and there were nodes with the purpose of providing content to other nodes. These server nodes typically have higher hardware specifications and are managed more closely by engineers. Since these nodes provide resources and services to others, they are typically referred to as servers. Servers typically sit idle, waiting for clients to initiate requests for their resources. This model of distributed resources that are asked for by the client is referred to as the client-server model.
Why is this important? If you think about it for a minute, the importance of networking is highlighted by this client-server model. Without it, there is really not a lot of need for network interconnections. It is the need to transfer bits and bytes from client to server that shines a light on the importance of network engineering. Of course, we are all aware of how the biggest network of them all, the internet, has been transforming the lives of all of us and continuing to do so.
How, you asked, can each node determine the time, speed, source, and destination every time they need to talk to each other? This brings us to network protocols.