Application Layer of the OSI Model: Definition, Functions & Protocols

Application Layer Definition

Application layer is the top most and 7th layer in OSI reference model which provide services for application program to establish effective communication with the other applications. Don’t go with layer name it is not about application itself rather layer is doing communication.

To read about OSI reference model click here.

Function of Application Layer

Now we are going to discuss some of the services provided by this layer are: this layer identify the other party, here the party is another application and layer is to make ensure it should be reachable by the others layers also. This layer authenticate either the message send or receive or both if required. This layer going to take care of necessary communication resources exist because the primary function is to establish communication. It enables the communication between network clients and services. Provide Encrypts and compress data and also specify the data format or file format.

Protocols of Application Layer


Telnet is application layer protocol used for remote logins to hosts means by running on your computer and connect it to server over the network. Then you have to enter some commands on the server console to run the program. This will enable the user to communicate to the other servers and control your own server. To start a Telnet session user need to enter username and password this is an authentication part.


FTP stands for File transfer protocol this is one of the standard protocol which is used to transfer the files between client and server over the network. This is the basic way to sending and receiving files by dividing the files into segment and assign each segment a reference id and send it over the network, while at the time of receiving receiver get the same copy of original one. It also perform automatic error detection and correction while transferring the file.


Stands for Dynamic Host Configuration Protocol, working of this protocol to assign a dynamic IP address to a device over a network. Advantage of using dynamic IP is device have different IP address so that every time it connects to internet.


Stands for Simple Mail Transfer Protocol primary purpose of this protocol is to transport electronic mail. Used for sending and receiving messages and ability to queue messages at the receiving end. This protocol usually used with one or more protocol like POP3 or IMAP. These protocol enable the user to save messages in the server mailbox and download them later.


DNS stands for Domain Name System is a dense protocol which do many things but the major one is to convert human readable hosts to IP address. When you find a web address on internet then your ISP views the DNS associated with that domain name you search and translate it into machine friendly IP address and directs you to the correct website.


HTTP stands for Hyper Text Transfer Protocol, This is the basic protocol used for World Wide Web. This protocol define how messages are formatted and transmit to user end and what appropriate action taken by server and browser to respond them.