This layer directly interacts with data from the user. The application layer handles the protocols and
data manipulation that the software relies on to present readable data. For example, HTTP and SMTP.

The application layer is used by network applications. A network application might be a web browser, outlook skype etc. These applications are dependent on application layer protocols to function. The following protocols form the application layer:

List of protocols @Mojca_Ciglaric

  • HTTP
  • FTP
  • NFS
  • FMTP
  • DHCP
  • SNMP
  • POP3
  • IRC
  • NNTP

These protocols form the basis for various network services such as file transfer, webserving, email, virtual terminals etc.
So, the application layer provides services for network applications with the help of protocols to perform user activities. It provides an interface for the user, think of your web browser.

Network Aware and Network Unaware

You may encounter these terms when asked about the application layer.

  • Network Unaware Applications:
    If an application program does not make any network requests it is considered a network unaware program.
  • Network Aware Applications
    These applications consist of two categories:
    • Programs that mainly run local but can, if necessary, connect with remote systems. Think of Word or Photoshop.
    • Programs that mainly work with remote systems. Examples include SSH, FTP, etc.

It is important to understand that only the second category of network aware applications are referred to in the application layer. This does not mean that applications such as Word or Photoshop do not fall in the application layer it just means they are not documented in the application layer.

The protocol data unit at this layer is called a message. This is the same for the Application, Presentation and Session layers.

Exam Question

A web browser requests a webpage from a web server. What is the correct order?

  • 1. The web browser requests the page using HTTP.
  • 2. The web browser establishes a TCP connection with the server.
  • 3. The server sends the page using HTTP.
  • 4. The browser resolves the domain name using DNS.

The correct answer is: 4, 2, 1, 3.

The browser first needs to discover what IP address needs to be retrieved. It does this using DNS. Then it establishes a TCP connection with that server and once this connection is established the browser retrieves the HTTP request. The server responds with the requested page using HTTP.

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To better understand the material it may be helpful to study exam questions. Here are some helpful resources: