A network is a collection of interconnected devices performing specific functions. These functions are performed at different places on the network. For example, controlling access to the network based on the user profile and credentials is done at the edge of the network; choosing one link out of multiple options to send traffic across the WAN is done at the WAN Edge; enforcing security policies to protect the server infrastructure of the company is done within the data center, and so on. This segregation of network functions helps to keep the network simple and modular.

Let's take a small organization as an example to discuss this concept in a bit more detail. Let's assume an organization ABC Inc. that starts in a single location. This organization will have a set...