Similarly, computer networks are conceptually divided into layers that each serves the layer above and below it.

For example, the top layer in most layered models is called the application layer. End-user applications live in the application layer, which includes the web and email and are almost always implemented in software. The application layer is also where an outgoing message starts its journey.

The application needs an underlying service that can get application messages delivered from source to destination and bring back replies which is what the layer(s) after do(es).

Since the underlying layer collects messages from the upper layer for delivery to the destination and hands over messages destined for the upper layer, it serves the application layer. Furthermore, the application layer abstracts, and hence is not concerned with any implementation details of the layers below