2.5 Stacking the Layers

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We usually show the four different layers (Link, Internetwork, Transport, and Application) stacked on top of each other with the Application layer at the top and the Link layer at the bottom.
The reason we show them this way is because each layer makes use of the layers above and below it to achieve networked communications.
All four layers run in your computer where you run the client application (like a browser), and all four layers also run in the destination computer where the application server is running. You as the end user interact with the applications that make up the top layer of the stack, and the bottom layer represents the WiFi, cellular, or wired connection between your computer and the rest of the Internet.
The routers that forward your packets from one to another to move your packets towards their destination have no understanding of either the Transport or Application layers. Routers operate at the Internetwork and Link layers. The source and destination addresses at the Internetwork layer are all that is needed for routers to move your packets across the series of links (hops) to get them to the destination. The Transport and Application layers only come into play after the Internetwork layer delivers your packets to the destination computer.
If you wanted to write your own networked application, you would likely only talk to the Transport layer and be completely unconcerned about the Internetwork and Link layers. They are essential to the function of the Transport layer, but as you write your program, you do not need to be aware of any of the lower-layer details. The layered network model makes it simpler to write networked applications because so many of the complex details of moving data from one computer to another can be ignored.
Up next, we will talk about these four layers in more detail.
In the early 1990s, as the Internet came to more people and computers’ abilities to handle images improved, the World Wide Web application was developed by scientists at the CERN high-energy physics facility. The web was focused on reading and editing networked hypertext documents with images. Today the web is the most common network application in use around the world. But all the other older Internet applications are still in wide use.
Each application is generally broken into two halves. One half of the application is called the “server”. It runs on the destination computer and waits for incoming networking connections. The other half of the application is called the “client” and runs on the source computer. When you are browsing the web using software like Firefox, Chrome, or Internet Explorer, you are running a “web client” application which is making connections to web servers and displaying the pages and documents stored on those web servers. The Uniform Resource Locators (URLs) that your web browser shows in its address bar are the web servers that your client is contacting to retrieve documents for you to view.
When we develop the server half and the client half of a networked application, we must also define an “application protocol” that describes how the two halves of the application will exchange messages over the network. The protocols used for each application are quite different and specialized to meet the needs of the particular application. Later we will explore some of these Application layer protocols.

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