Application Layer

The Application Layer is the layer closest to the end user. When you are using a program that is going to send something or contact someone over the internet you are using a network application that operates at the Application Layer. Each program/application that sends data over a network is identified by a particular protocol, at Layer 4 this protocol is associated with a port number. For example a web browser like Internet Explorer requests and receives pages from web servers its protocol is HTTP the hyper text transfer protocol and its correlative port number is port 80.

The applications that we use at Layer 7, the Application Layer are web browsers (HTTP) like Internet Explorer, a file transfer programs (FTP) like Filezilla, email clients (SMTP) like Microsoft Outlook Express and all flavors of Instant Messaging programs and P2P applications. There are also processes that run in the background that run at Layer 7, like DHCP which automates the process of requesting and receiving an IP address from a DHCP server. If you want to see these protocols in action all you need to do is use Wireshark. For instance, if we use the example of DHCP we learn that initiating DHCP involves a DHCP client talking to a DHCP server. The process is: 1. Client sends a DHCP DISCOVER 2. Server responds with a DHCP OFFER 3. Client sends a DHCP REQUEST 4. Server responds with a DHCP ACKnowledgement You can see the process of a client obtaining an IP address with DHCP in Wireshark (see video tutorial below).

This is a diagram of the OSI and TCP/IP Models and how they correspond to PDUs, Protocols and Devices  

This is a visual diagram of the process data goes through when sent over a network in a layered architecture