Application Layer

The Application Layer (layer 7) is the highest layer in the OSI reference model (although you may sometimes hear coworkers joke about end users being the theoretical eighth layer). This is the layer that most end users of networks and systems are familiar with, as it is responsible for directly providing services to application processes or programs that we use on a day-to-day basis. This layer also encompasses any other services that are not provided by the lower layers. This includes functions of programs, as well as end users (the people using these devices). As the highest layer in the model, the Application Layer provides the means for processes and end users to access and interact with the OSI protocol stack. Applications and protocols that we utilize in our machines (such as web browsers and email clients) are categorized in this layer. Devices such as desktop computers, mobile phones, and special layer 7 or Application Layer firewalls operate at this layer. Common protocols at this layer include X.500 (used to provide electronic directory services) and X.400 (a message handling system).