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It takes an amazing amount of software and hardware working together to make a network function.
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So the challenge to all of this is that if you have different pieces of software and hardware they don't
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have to be able to talk to each other and interact and able to make your network go.
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You don't have a network card from 3Com and you might be running a Windows 8 operating system and all
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of these devices have to work together. To help make interconnectivity work
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they came up with something a long time ago called the OSI seven-layer model. The OSI seven-layer model is seven
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distinct functions that a network must do.
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Now the OSI is being supplanted with something called the TCP model.
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So, let's do a quick overview and take a look at OSI and TCP. Now for OSI we're going to start,
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basically there's no real bottom or top but I tend to look at it at the bottom which is your most basic
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part which is what we call the physical layer. The physical layer is, that's like the easiest part of
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the OSI seven-layer model.
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It's just what type of cables do I use and things like that.
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Now coming up from that is the data link layer.
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Now the data link layer a little bit more important.
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Basically anything that works with a MAC address works at the data link layer.
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So when we're talking about data link we start talking about things like the network cards themselves,
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start talking about switches things like that.
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The third layer is the network layer.
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Now the network layer is important because the network layer has to do with logical addresses.
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So when you see the network layer you're talking about IP addresses.
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So things like routers in particular work at Layer 3. The next layer up is the transport layer the transport
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layer is kind of interesting.
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Data is big,
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and the actual chunks of data that go through a network are relatively small.
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So there has to be a assembly disassembly area for data as it gets broken down into individual packets
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and that's what the transport layer is all about. The transport layer
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not only disassembles packets but make sure the packets get to the other system in good order.
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Next one is the session.
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Now the session is kind of interesting. When we work at a session layer here we're talking about the
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actual connection between two systems.
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Are we making a TCP connection between a web server and a web client?
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Are we just sending e-mail?
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Are we sharing a folder between two computers on a local network?
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And it's the session layer that defines what's taking place in terms of how that connectivity really
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works.
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Next is the presentation layer. The presentation layer is old and weird, but since it is on the OSI we
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need to at least make a quick mention of it. Just because a big chunk o data comes into your computer
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does that necessarily mean that this data is in a form that your computer can read it?
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Now, let's use Microsoft Office as an example.
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If I open up my Microsoft Office I can open up almost any kind of document you can dream of.
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Office has the smarts built into it to automatically read it.
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But that wasn't always the case.
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So the presentation layer used to be used to convert data into a format that your applications can read.
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hippy you're at the top is applications.
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Applications is important because it's the applications themselves and it's not really the applications
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you have to be careful here.
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What we're talking about is the smarts in the applications that make them network aware. Little simple
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things that you and I never think of. For example, if we were to open up Microsoft Word and we could literally
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go to the network to go find a particular file.
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And that's because Microsoft Office itself is network aware.
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We often use the term API, or application programmers interface, as the definition of the smarts that
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are built into an application that allows us to make the application network aware.
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So this is the OSI. Alright, the TCP/IP model is more simplistic than the OSI simply because if you're
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using only TCP/IP there's a lot of parts don't really come into play as much.
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Let me show you what I'm talking about.
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First of all we have what they call the network interface layer. So the network interface layer covers
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all the physical cabling, MAC addresses, network cards, pretty much everything in terms of hardware with
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a couple of small exceptions, routers, are all covered here under network interface. So that ties similarly
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to the data link and physical for the OSI.
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Now your next one is your internet link.
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Your internet for IP addresses. So things like routers or anything it has to do with an IP address
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works at the Internet layer.
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The next one is transport.
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Now transport does all the assembly and disassembly but it also has to do with whatever it takes to
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connect to the other system to make sure the data gets there.
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So in this case we're talking about things like, for example TCP or UDP or whatever it takes to get those
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chunks of data from one application to the next.
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Now the last one is called application.
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So application actually takes into consideration the old OSI application, presentation and session layer.
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So everything that has to do with the application itself works at this layer.
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Now keep in mind that the TCP/IP model looks at this a little bit differently than OSI.
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TCP model looks at applications as applications for example email or FTP or telnet.
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All of these very distinct things that each have their unique port numbers all play into this one particular
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area.
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So if it were up to me, and it's not but to me, I would change this a little bit.
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First of all I would go ahead and take the transport layer and put the old OS I session layer into that as
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well as the transport layer.
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But that's really not the way we look at it for the exam so we put that back.
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And that's pretty much your TCP/IP model.
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Now what I want to do is let's take a look at both of these guys.
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First we have the OSI model, older more detailed.
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And then we have the TCP/IP model, more modern,
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not as much detail but not as complicated.
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Make sure for the network plus exam that you know both of these models. Now don't worry about it.
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We've got a lot more to talk about.
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We've got lots of episodes that are going to break down all the different layers in these models and
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a lot more detail. But for right now the important thing you need to appreciate is that each one of these
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layers has a distinct function.
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Each one of these layers has a name but not only that it's also got a number.
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So let's use OSI as an example.
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If I'm talking about OSI and I'm looking at the network layer. Now understand its network layer but
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there's also numbers associated with it.
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They go from one up to seven.
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So if I'm talking about OSI and I say Layer 3, you need to know that I'm talking about the network level.
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That's also very true with TCP/IP.
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So whenever we're talking about the TCP/IP model there are numbers associated with it.
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Make sure for the network plus you know each name of each layer as well as the number associated with
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it.
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