App layer

10.   Application layer
The application layer contains the higher-level protocols used by most applications for network communication. Examples of application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as TCP or UDP), which in turn use lower layer protocols to effect actual data transfer.

As the IP model does not consider the specifics of formatting and presenting data, it defines no layers between the application and transport layers as in the OSI model (presentation and session layers). Such functions are the realm of the applications which often implement such functions in libraries and application programming interfaces.

Application layer protocols generally treat the transport layer (and lower) protocols as black boxes which provide a stable network connection across which to communicate, although the applications are usually aware of key qualities of the transport layer connection such as the end point IP addresses and port numbers. Transport and lower level layers are largely unconcerned with the specifics of application layer protocols. Routers and switches do not typically "look inside" the encapsulated traffic to see what kind of application protocol it represents, rather they just provide a conduit for it. However, some firewall and bandwidth throttling applications do try to determine what's inside, as with the Resource Reservation 

Protocol (RSVP). It is also sometimes necessary for network address translation (NAT) facilities to take account of the needs of particular application layer protocols.

The TCP/IP model's application layer encompasses the fifth, sixth, and seventh layers of the Open Systems Interconnection (OSI) model, which are the session layer, presentation layer, and application layer, respectively.