As we all know , network internet model has five layers: physical layer , data link layer , network layer, transfer and application layer. The OSI model has seven layers and we are not going that way. This tutorial will cover some of the basics of application layers today. Network application development refers to the development of some programs that will work in the application layer and which will run on different end systems ( end system- network end system or end user system) and will be able to communicate with themselves or other systems. An example of this is a simple web application where two different programs communicate with each other. The first is the browser program that runs on the user host (desktop , laptop , PDA , mobile, etc.) and the second is the web server program that runs on the server computer. Another example of network application development is P2P file sharing or peer-to-peer file sharing. Here, each host runs a program , through which file sharing is done between many PCs, such as Kaza , Aimesh, etc. These programs running on different heats may be the same or different. It is important to note that network application development does not require the creation of software that will run on the core devices of the network , such as routers or Ethernet switches. This is because the application does not run on layers on these devices, it runs on the network or the layers below it.