The Application layer is the top layer of the OSI model, and is considered to be the place where the user interacts with the network. This interaction usually occurs by running a program, such as a web browser or a spreadsheet application. This layer doesn’t really worry about the network. Instead, it simply knows how to make a request for something (this is the data), and then what to do with the reply (the data that was requested). In the case of a user browsing a website, the client application (the web browser) makes an HTTP request that will be understood by the receiving application at the other end (the web server). This is sometimes referred to as program-to-program communication. The lower layers of the model concern themselves with how this data is actually encapsulated and transmitted.
Examples of common Application layer programs and services:
- Word Processor: Creating documents, possibly to be saved to a network server.
- Web Browser (HTTP): Access to internet web services
- Email Client (SMTP, POP3, IMAP): Sending and receiving email
- Telnet: Remote terminal session
- File Transfer (FTP): File transfer
Tip: Take the time to familiarize yourself with the various protocols and applications that exist at the different OSI layers.