The Application Layer is used by most programs for network communication. Data is passed from the program in an application specific format, then encapsulated into a transport layer protocol. Internet Protocol stack has no layers between application and transport layers. The application layer must include any protocols that act like OSI reference models presentation and session layer protocols. This is done through libraries. Data sent over the network is passed into the application layer where it is encapsulated using the application layer protocol.
From there the data is passed down to the lower layer protocol of the transport layer. Two most common end-to-end protocols are TCP and UDP. Common servers have specific ports assigned to them (HTTP has port 80 and Telnet has port 23 etc.) While clients use ephemeral ports. Some protocols such as file transfer protocol (FTP) and Telnet may set up a session using a well known port but then redirect the actual user session to ephemeral ports. Routers and switches do not utilize generally this layer, but some protocols like resource reservation protocol do use application layer.
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