Layer 7 of OSI model known as OSI Application Layer is the closest layer to the end user. The users interact directly with this layer with the application layer software that has a two form: applications and services. Applications are designed to interact with users and services are background programs that perform a particular function in the data networks.
OSI Application layer used protocols that are implemented within application and services. Application layer protocols must be used during a communication session at the source and destination devices. For successful communication the Application layers protocol implemented at the source device and destination device must match. Protocols have a function:
• Establish consistent rules between application and services at the source and destination device for exchanging data.
• Specify data types and structure that are sent between participating devices.
• Implement multiple protocols for specific communication; one protocol may specify how to initiate the connection and another for the process of the data transfer.
Basically OSI Application Layer chooses the proper application base on the Transport layer address called port number. Server programs generally use predefine port numbers that are commonly known by the clients. Some example of Application Layer services such File Transfer Protocol (FTP) port 20 and 21 TCP, Telnet port 23 TCP, Simple Mail Transfer Protocol (SMTP) port 25 TCP, Dynamic Host Configuration Protocol (DHCP) port 67 and 68 UDP, Domain Name System (DNS) port 53 TCP/UDP, Hypertext Transfer Protocol (HTTP) port 80 TCP, Post Office Protocol (POP) port 110 TCP.