Networks are broken up into multiple layers, they can get confusing, how is it possible we can remember the differences between a network interface card and a network cable, they seem different but they are classified the same. Why is that? What exactly makes things classified as "different?" Well this is where and why the OSI model was created, also known as the Open System Interconnection Model. A layered approach to classification of items in a network to more streamline their design and purpose.
If you haven't noticed I keep saying the words "layer" over and over and over. In the post "What's An IP Address (part1)" we frequently refered to physical items as layer one, switches as layer two, ip addresses as layer three. But really what is all of this? What does it all mean? Truth be told the OSI model is a subject of deep debate and we could spend DAYS talking about what it is, why it is what it is, how it works. Lets start to break down what each layer does, and then revisit why we use this model in the first place.
Layer 1
Layer 1 is the physical layer, at this layer we are talking the physical level (obviously) and electrical level of the components required to make a network work. Inside the physical layer we see very basic and rudimentary devices and functions. The physical layer talks bits, decides if things are full duplex or half duplex, copper wiring or fiber optic, network interface cards and wireless adapters.
Layer 2
Layer 2, also known as the Data Link layer is responsible for node-to-node data transfer, a direct connection between two devices. At this layer you'll find your MAC address, which is your Media Access Control. The MAC address is registered on both ends for a secure connection. Layer 2 houses switches and the burnt in ID (your MAC address).
Fun note, switches and routers have MAC addresses, but hubs do not, another reason they are at layer 1.
Layer 3
Layer three, known as the network layer, is where IP addresses are actually used, it defines where the interconnected network starts and stops, where Layer 2 sets the boundaries. On the network layer this is where reassembly of fragmented data is performed, encapsulating packets occurs, and routing.
Layer 4
Layer four, also known as the Transport Layer is where network connection protocols are decided. Protocols such as UDP (user datagram protocol) or TCP/IP (transmission control protocol/Internet protocol).
Why not 5, 6, or 7?
This article is focused primarily on the network itself, all things above layer four are not really network oriented by session oriented within the systems themselves. What application is running, how that application is being presented and maintained.
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