Application layer


As I am sure, most of you use the internet in a number of ways; be it browsing the web, downloading movies, financial transactions, blogging, IM messaging among others. We are most likely oblivious of what it is that makes this communication possible.

In this chapter, we will look at the first layer of the TCP/IP and OSI models, which is the application layer.

In this chapter, you will learn to:

  • Describe the first 3 layers of the OSI model
  • Describe some of the application layer applications


The application layer is the 7th layer in the OSI model. When communicating, messages move from the application layer, down the other layers up to the physical layer, after transmission, the information moves from the physical layer up the model to the application layer again from where it is usable.

The application layer in the TCP/IP model defines 3 layers in the OSI model. These are; the application, presentation and session layers.

Presentation Layer

The presentation layer is the 6th layer in the OSI model. It defines three main functions when it comes to communication over the network.

  1. Data from the application layer is converted and coded into a form that can be understood by the recipient’s device.
  2. Since application data may be uncompressed, the presentation layer compresses the data and this is in a manner that can be interpreted and decompressed by the destination device.
  3. To ensure security of the data which may be transmitted over insecure networks, the presentation layer also encrypts the data.

In this course you are not expected to fully understand the functionality at this layer, however, you are supposed to be familiar with the processes that are involved as listed above.

The Session Layer

Communication over networks can be likened to moderated debates, as such, it is the work of the session layer to make sure that the communication is kept alive, some of the functions include; starting dialogs, keeping them active, and where necessary restart them if they are inactive or idle.

NOTE: Most applications, like web browsers or e-mail clients, incorporate functionality of the OSI layers 5, 6 and 7.

TCP/IP application layer

The application layer as defined by the TCP/IP protocol suite defines the user data. Some of the most common applications that you may know of include the following:


  • Hypertext Transfer Protocol (HTTP) and Secure HTTP (HTTPS) are used to transfer web pages from the web. Whenever you browse or access web pages, these are the protocols involved.
  • Email is an important component in communication in not only enterprises but also individuals. The two most common protocols that are used in the transfer of email messages are the Simple Mail Transfer Protocol (SMTP), and the Post Office Protocol (POP).
  • Transferring of files between different devices is also important, for this we use the FTP (File Transfer Protocol).


The application layer is the interface between the end user and the hidden or underlying network devices. The protocols defined here are used by both the sender and recipient and they must match for communication to work. For example, when sending an email, both the sender and the recipient must have SMTP and POP.

There are several protocols at this layer, however, they all define the following:

  • Processes on both ends of a communication stream.
  • They define the message type that is being sent.
  • They define the format or the syntax of the messages being sent and received.
  • They have fields meant for control and additional information on the message.
  • They specify the type of expected response to a message that has been sent
  • They specify how the application layer will interact with the transport and other lower layers.

There are many application layer protocols that have been implemented on networks. We will explore how we control these protocols in future chapters, however, you may want to explore further the various common protocols for both the CCNA exams as well as practical use in the field.


In this chapter, we have looked at the application layer and how it plays a critical role in communication. In the next chapter, we will discuss the transport layer and its role in communication. There is much more information on the application layer not encompassed in this chapter, therefore, further reading is recommended. The concepts learnt in this chapter, will be applied throughout the course as well as in the certification examinations.

In the next chapter, we will look at the transport layer and discuss how it is crucial in communication.