Application
Layer

Overview

The Application layer, layer 7 of the OSI Network Reference Model is open to misinterpretation. Application layer protocols do not include end-user application programs. Rather, they include utilities and networked-based services that support end-user application programs. Some people include network operating systems in this category. Strictly speaking, the best examples of Application layer protocols are the OSI protocols X.400 and X.500. X.400 is an open systems protocol that offers interoperability between different email programs and X.500 offers email directory synchronization among different email systems. The Domain Name Service (DNS), which is an Internet protocol that resolves computer's common or domain name to a specific IP address, is also considered an Application layer protocol. Other Application layer protocols include: HTTP, SMTP, FTP, NNTP, Telnet, TFTP, and SNMP to name a few.

The Application layer provides the interface to the user software.

The PDU at the Application is referred to as data, messages or streams.