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Frames are a fantastic way to make data move around.
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The problem is is how do frames know how to get to the right computer?
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So if you take a look at a network what we have here is a hub.
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Now on this hub, if I'm sending data it's coming in from the white cable when it gets inside the hub.
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The hub is what we call a repeater.
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It takes the signal that comes in from one of the ports and then it recreates multiple copies of that
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and sends it out on all of the other connected cables.
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So, so it literally takes one signal and repeats it out to as many connections as you have.
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Now that creates a big problem.
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The challenge we have here is I've got a network.
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All right so this is going to be for computers that are connected to a single hub.
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The challenge that we have is that let's say that this purple computer wants to talk to the green computer.
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What's going to be taking place is that when this purple computer sends out a frame it gets sent into
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the hub and the hub remember repeats it out to all of the connections so everybody gets an example of
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that frame.
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Now that frame isn't for everybody it's just for the green computer.
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So we need some way to address each of the frames so that even though everybody gets it only the green
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computer is actually going to read the frame and use it.
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These other two guys when that frame comes in they're going to see that it's not for them and then they'll
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just wipe it out or consume it so it never goes past the network card.
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So in order to do that I want to make an analogy of a network card as a tray.
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So this for me, I'm have a little fun here, is a network card. This ugly old vacuum tube from my shop vac
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is the RJ45, the wired connection into it.
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So when a frame comes in it literally plops down and the network card looks at it.
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Now the problem is there's nothing that identifies it and says it's for this network card for this particular
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machine.
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And that's where something called a MAC address comes into play. In order to appreciate a MAC address
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we're going to have to take a look at a computer.
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So what I'm going to do is fire up my windows computer and let's take a direct close personal look at
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a MAC address.
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So here I am in Windows 10 and when I'm going to do is I'm going to fire up PowerShell.
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Now a lot of people are like 'oh PowerShell why can't we just use a command prompt?"
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You can.
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All right.
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We could.
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What I'm about to do works equally well in a command prompt or in PowerShell.
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I just like PowerShell so let's just do it here.
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So I'm first want to teach you an incredibly important command that you're going to use like crazy and
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it's called ipconfig.
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So i p c o n f i g.
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Now if I type it by itself we're going to get some kind of information but I want you to type it a special
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way. What you're going to do is type ipconfig slash all just like that.
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Now when we type that we can see all the information there is about all of our different network cards.
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Now this is the card I'm interested in because this is the one I'm actually connecting to the network
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on. it's just a regular Ethernet NIC.
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And what I want you to look at is right here. You see this? So we have 12 numbers here broken up into
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pairs.
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So there are six pairs.
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This is our MAC address.
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So this is the unique identifier for the network card.
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Now as we take a look at this, first of all this is a 48 bit address,
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we know that these are hexadecimal values and there's 1 2 3 4 5 6 7 8 9 10 11 12. Each hexadecimal character
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represents four binary characters.
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So four times 12 is 48.
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Now if we take a look at these first half. These first three pairs these are known as the OEM numbers.
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Intel, the maker of these network cards, is issued this value from the Internet folks.
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So these numbers right here are issued to Intel.
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So every Intel NIC,
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well actually Intel make so many they have a number of these, is, use that unique OEM identifiers we call
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it. The last six values are burned in to each card at the factory and each card gets a different value.
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So we have the OEM, whoops, the OEM
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and then what we call the unique ID.
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So the thing we need to remember is that every network card in existence has a unique MAC address.
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It has to have that because we never know on our network here whose network cards are going to be plugged
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in.
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And it's the MAC addresses that we apply to the frame to make sure it gets delivered to the right place.
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So each one of these computers has a unique MAC address.
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So what we'll do,
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let's take a look at our frame again, is we're going to add to the frame MAC addresses.
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Now I've got two blocks here and that's because we have the MAC address where it's going to and the
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MAC address of where it's coming from.
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So whenever your network card sends out a chunk of data it's very important to the network card that
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not only does know where to send it to but it also will put its own MAC address in there so the receiving
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computer can send it back.
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Now the other thing that will come into play here is a CRC or a cyclic redundancy check.
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This is just used as a way to verify that the data is good. If it's bad data then it knows to resend
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it.
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So what we'll do, let me go and get it pointed in the right direction, is once this is all created it gets
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sent out.
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I'm not going to push it through too far because I'll never get it back! But push, off it goes off to the
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network.
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Now the cool part to all this is that as it leaves this computer and comes into the hub, remember that
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the hub creates as many copies as necessary to represent all the different computers it's connected
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to.
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So it makes, in this case, one to three new copies and it sends them down the line to all the individual
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computers,
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and as these frames come into the computer, it drops down on the tray, and the cool part is is that every
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network card knows what its MAC address is.
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So the card looks at this.
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If it's a MAC address for him then it's going to strip away all this extra information and send it up
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into the software of the system.
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However if it's a MAC address that's not for him he'll look at it see it's not his MAC address and he
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just makes it disappear and doesn't do anything with it.
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And that is how MAC addresses work.
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