The application layer is layer seven of the OSI model. This model is a representation of guidelines governing network design. This is the top layer of the model (it also rests at the top of the TCP/IP model). This is important to note because information in the OSI model starts from the top before being transmitted to other layers. This means that data transfers begin at the application layer.
So, what does all this really mean? Well, the main idea behind the application layer is that it gets communication from humans ready to be transmitted across the data network. Sound confusing? Maybe, but it really isn’t so bad. Let’s think of it in more specific terms.
Any time that you send and receive data across a network, you are using the application layer. This means, that when you use your web browser, for example, you are using the application layer. The application layer protocols are what make communication through the Internet as we know it today possible! When we use applications that involve the application layer, we make contact with other servers and clients through application layer protocols.
Some of the popular application layer protocols are: HTTP, HTTPS, POP3, FTP, SSH, TELNET, TFTP, SMTP, DHCP, DNS, and NTP. These protocols layout the guidelines for network communication. If you frequently use the Internet, for example, you have probably seen HTTP and HTTPS. POP3 and SMTP are both crucial to the sending and receiving of emails.
As you can see, communication occurs at the application layer. Without this layer, we wouldn’t be able to perform some of the most basic functions on the Internet as we use it today. Another way in which the application layer does this is that it makes sure that all the components needed for communication are present. This includes making contact with senders and receivers and making sure that the necessary nodes are in place for the communication to take place.
For example, if you’ve ever pulled up your web browser only to see that you have failed to connect to the Internet, you are seeing a message that there has been a failure of application layer protocols to establish the necessary connection to get you online.
As you can see, the application layer is extremely important because when all of the application layer protocols are working as they should, we are able to enjoy most of the applications we use on computers and other devices.