The reference models demonstrated a standard way for data communication between two nodes. Of course by now, we all know that not all nodes are created equally. Even in its DARPA-net days, there were workstation nodes, and there were nodes with the purpose of serving content to other nodes. These units typically have higher hardware specification and are managed more closely by the engineers. Since these nodes provide resources and services to others, they are typically referred to as servers. The servers are typically sitting idle, waiting for clients to initiate requests for its 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 really based on 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, is 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 each time they need to talk to each other? This brings us to the network protocols.
