Application Layer of OSI Model (Layer-7)

Application Layer is the highest layer in seven layer OSI Model. It is responsible for End user Network Applications Management. This is the layer where real traffic data is generated in most of the cases. For example, this could be a web request generated from HTTP protocol or a command from telnet protocol. OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. The seven Layer model starts from Physical Layer & goes till Application Layer on the top, as below in figure:

Functions/Duties of Application Layer

Each Layer in OSI Model Performs some important duties. Important functions performed by Network Layer are listed here:

Application Layer manages applications software running on a computer or terminal. This is the most important function of Applications Layer.
It works like an interface between Network Services & applications. Application Layer supplies Network Services to End User.
It is also responsible for End User Processes Management.
Miscellaneous other services are also provided by application including Naming Services (e.g. DNS), Mail Services (e.g. SMTP/POP3), Remote Login Services (e.g. Telnet/SSH), Remote Host Management (e.g. SNMP) ,…

Application Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Physical layer are listed in below:

HTTP (Hypertext Transfer Protocol)
DNS (Domain Name System)
DHCP
FTP (File Transfer Protocol)
Telnet
POP3/IMAP
BOOTP (Bootstrap Protocol)
Bitcoin, …

Network Equipment/Components at Application Layer

Firewalls
Gateways
End Devices e.g. Computers, Smart Phones, Servers, …
Devices that inspect & balance application traffic e.g. Proxies, Firewalls, Load Balancers …

Summary

Application Layer is the 7th Layer of OSI Model. It performs important functions like Applications Management, End User Processes Management & Miscellaneous other services like DNS, DHCP, SMTP and Telnet. Important Protocols at Application Layer include HTTP, DNS, DHCP, FTP, Telnet, POP3, IMAP, BOOTP and Bitcoin. Equipment operating at Application Layer include End Devices, Firewalls, Gateways and Load Balancers.

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