We experience the Internet through the World Wide Web when streaming videos, playing online games, chatting with and emailing friends, and shopping for deals on web sites. Applications, such as the ones used to provide the services mentioned, provide the human interface to the underlying network. They enable us to send and receive data with relative ease. Typically we can access and use these applications without knowing how they work. However, for network professionals, it is important to know how an application is able to format, transmit and interpret messages that are sent and received across the network.

Visualizing the mechanisms that enable communication across the network is made easier if we use the layered framework of the OSI model.

In this chapter, we will explore the role of the application layer and how the applications, services, and protocols within the application layer make robust communication across data networks possible.