Hello in this section i'm going to review some of the fundamental concepts of networking the core concepts that are really important to understand in order to be able to design and run a network effectively now to do this i'm going to refer to this diagram which you may have seen before it's the osi open systems interconnect model seven layered model and it breaks down networking into these individual parts called layers now each layer provides a service to the layer above and it makes use of the services of the layer below to do its job except for layer 1 which is the bottom and therefore it doesn't have any layer below it so let's look briefly at the function of the layers layer 1's job is very simple it's just to take a stream of zeros and ones and transfer that across some medium like a copper cable or a fiber optic and convert them back into a stream of zeros and ones at the other end hopefully the same stream layer two is called the link layer or the data link layer and its job is to organize that stream of zeros and ones into frames meaningful units of information and those frames may also contain addressing information so that a frame can be received by one particular device but ignored by other devices and it may also contain some error detection information now layer 2 is what we use for building local area networks are lands layer 3 is the network layer or internet world layer now its job is to combine multiple layer 2 networks together to join them together so that data can flow from one network to another network to another network to reach its final destination and it's layer 3 that allows us to build a global scale wan in other words the internet layer 4 adds the ability to address data to a specific process running on a machine and layer 4's job can also include taking large amounts of data breaking them up into smaller datagrams that are suitable for layer 3 to deliver and to reassemble them and also to re-transmit data when there's an error layers five and six you don't have to worry about because they're not actually used in the internet suite and layer seven the application layer is the actual job you were trying to do in the first place so for example a web client tries to fetch a web page from a web server or an email server wants to deliver an email message to another email server and at layer 7 there are their own application protocols for doing each of those different jobs so by putting the layers together you've taken a complex job such as reliably transferring a web page from one side of the world to the other and broken it down into smaller pieces that can work together to perform that job now this layered model is not just some theoretical model it very much demonstrates exactly how the networking is working and if you look at packets being transferred across a wire with a tool that will read and analyze them such as wireshark you can actually see different parts of the packet that belong to each of these different layers each doing the jobs that those layers need to do and so using this layered model helps you to design your network right so that it can work reliably and it can scale

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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