WEBVTT

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 So in the previous video, I talked about
 why we would need to have routing

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 in a network if you want to break it
 up into multiple broadcast domains.

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 And I talked about the concept of what
 is a routable address and that

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 a device needs to be able to identify
 its routable address.

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 It needs to be able to figure out from
 that address what network it belongs

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 to and it do with packets that
 don't belong to my network.

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 What do I do with them?

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 So I want to show you something here
 with Windows 10 and this will be

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 the same thing with
 Windows 7 as well.

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 If you go to your DOS prompt, so just
 click on start and then type in

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 CMD for commands, CMD or any other way
 you have to get to your DOS prompt.

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 I'll make this a little
 bit larger here.

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 So first of all, how do you see what
 the routable address is that your

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 laptop has? You can type
 in the CMD IP config.

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 Let me just see real quickly if there's
 a way I can make this a little

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 bit larger. Oh, font, here we go.

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 This will help. There we go.

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 Okay, so you can type IP config.

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 I'm going to go to my wireless adapter
 because that's why I'm connected

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 to via right now.

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 So with IP config, you can see here,
 here's my wireless adapter and IP

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 is a layer three routable
 protocol.

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 So we said IP config.

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 So this is dealing with
 our routable protocol.

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 So this is the routable address
 that that protocol has given me.

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 So my laptop says, okay, in addition
 to my layer two non routable address,

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 which is my MAC address, here's
 my routable address.

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 And by using that in combination with
 the subnet mask, by pairing these

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 two up together, it can extract from
 that what network it belongs to.

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 In this particular case, it knows that
 it belongs to the 10.7.1 network.

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 So every other device in my room, in my
 network, has an IP address belonging

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 or beginning with 10.7.1.

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 Without the subnet mask, this routable
 address would be useless.

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 There'd be no way to extract from
 it what network it belongs to.

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 And this particular case, the .11,
 that is my unique serial number.

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 That is my unique name
 in the 10.7.1 network.

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 So that answers the question of what
 is my routable address and how do

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 I extract from that
 what network I'm on.

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 So if I told my laptop, if I open up
 some programmer application that

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 said, okay, I want you to send
 some data to 10.7.1.5.

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 All right. So if I said 10.7.1.5, it
 would say, okay, first thing I'm

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 going to do is I'm going to answer the
 question, is that destination in

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 the same network as me or is it in a different
 network, a different broadcast

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 domain. So it'd say, okay, well,
 my IP address was 10.7.1.1.

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 And it took this, no, 10.7.1.11.

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 And it took the subnet mask
 and it put it underneath.

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 And anywhere there's a 255, that means
 that the entire digit right above

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 it belongs to your network.

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 So all of 10, all of
 7, and all of 1.

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 So that's how my device learned.

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 It said, okay, of my subnet mask, it's
 telling me that the first three

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 digits out of four belong
 to the network.

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 So now I'm saying, okay, well,
 I want you to go to 10.7.1.5.

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 Well, it does, it says,
 okay, I'm 10.7.1.

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 Let me compare the destination.

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 Oh, it's also 10.

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 It's also seven.

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 It's also one. I've got my answer.


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 This destination is in the
 exact same network as me.

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 So no routing is necessary.

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 So now he'll say, okay, well, let
 me shout out into the network.

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 Hey, 10.7.1.5. I know you're
 in the same room as me.

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 I know you're in the
 same network as me.

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 But I need what your non
-routable address is.

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 I need your layer two non-routable address,
 your name, your unique name.

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 So I can talk to you.

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 And the device would come back and say,
 oh, well, my non-routable address

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 is, and then it would
 give its MAC address.

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 And so now I can talk
 to it directly.

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 As opposed to if I had said, well,
 I want you to send instead a packet

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 going to 10.7.3.5.

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 It would say, okay, 10.7.0.3.

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 That's not the same as me.

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 Therefore, this is a
 different network.

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 I don't know where it is.

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 Could be five feet away.

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 Could be 5,000 miles away.

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 All I know is it's not in my room.


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 It's not my directly
 connected network.

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 So that brings us to the question
 of, well, what do you do then?

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 Well, a routable protocol like the Internet
 Protocol has to answer that

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 question. What do you do when the destination
 is not in your network?

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 You say, well, a host device, all a
 host device knows about your laptop,

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 your smartphone, your tablet.

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 All it knows about is its own network
 and how to talk to people in its

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 own network. It's like saying, okay,
 you're in a room full of people.

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 You know that to talk to people in
 the room, you talk to them directly

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 or you shout out, hey, is there anybody
 over here who's wearing a red

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 shirt? But you talk
 to them directly.

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 But now the question is, how do you talk
 to someone if you know what their

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 name is and you know they're
 not in your room?

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 You know you can't talk
 to them directly.

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 Well, the Internet Protocol says you're
 going to use an intermediary.

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 You're going to use a device.

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 You can talk to that device and that
 device will relay or convey your

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 information to who it is you want
 to talk to, to your destination.

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 And that's what we call a default gateway
 in the world of the Internet

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 Protocol. It's called a router
 or a default gateway.

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 So a default gateway is a device that
 sits in your room with you in your

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 network. You can talk to it directly
 just like you talk to everybody else

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 in your room. But what makes him unique
 is that he knows about other networks.

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 He knows about other rooms.

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 So if I need to talk to Pablo and I realize,
 oh, Pablo is not in my room.

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 He's somewhere else.

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 He's in some other network.

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 Then I'll send my information.

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 I'll talk at layer two to my default
 gateway and I'll say, hey, default

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 gateway. Yeah, Keith, I have some information
 here that needs to go to

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 Pablo. Can you get this to him
 and then he'll send it on.

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 So in the world of the Internet Protocol,
 we also need a default gateway.

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 And that's what we see right here.


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 Ten seven one one is
 my default gateway.

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 No, it's ten seven one.

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 It's in the exact same network as me
 ten seven one, which means I can

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 talk to him directly.

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 If a device had an IP address of a default
 gateway, but the default gateway

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 was not in the same network,
 that wouldn't work.

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 That would break the rules of IP.

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 There's no point in
 having a default.

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 It's sort of like saying, okay,
 you're in the basement.

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 And if you want to talk to anybody in
 the outside world, you have to talk

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 to John who's on the
 who's in the attic.

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 But I'm not going to tell you
 how to get to the attic.

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 I'm not going, you know,
 you're in the basement.

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 You know how to walk around the basement,
 but I'm not going to tell you

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 how to get to the attic.

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 But that's where John lives who can
 get to the rest of the world.

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 What good would that do you?

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 It would do you no
 good whatsoever.

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 So your default gateway has to
 be in the same room as you.

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 Otherwise, he can't serve
 his role, his function.

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 Now, one other thing I want to show
 you here is in your output, you can

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 type route print.

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 And what this shows you is you can
 see here you're, and I'm just going

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 to focus in right now on IP version
 four, is your routing table of your

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 laptop. Now, I want to, I'm going to
 get back to that for a second because

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 saying that this is a routing table
 is a little bit of a misnomer.

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 But the way you would read this is this
 says, okay, for any packet that

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 this device creates, any time my laptop
 creates a packet, going to x.x

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.x.x is the destination.

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 So that's the destination
 IP address.

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 It's going to say, okay, I'm going to
 look in this table and this table

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 is going to tell me what
 to do with that.

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 So for example, if I create a packet
 going to 10.7.1.30, well, it would

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 go through this table
 and find a match.

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 It says, okay, here's 10.7.1, and this
 subnet mask says match the first

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 three numbers. Okay,
 that would match.

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 Is there anything even more
 specific than that?

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 Well, let's see here.

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 Not that matches this, 10.7
.1.11, no, 10.7.1.255, no.

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 So this is my most specific
 match right here.

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 So my route table in my laptop says, okay,
 if my packet matches this entry,

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 then I need to send it to 10
.7.1.11, which is on my link.

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 It's on my ethernet cable.

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 So this is where I would
 forward that packet.

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 Now, if I gave it another address of,
 let's say, eight, eight, eight,

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 eight, he'd look through here and he'd
 say, well, of that, this is the

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 only one that matches.

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 This says you don't have to
 match any of these bits.

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 This basically is like a
 match any or a match all.

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 It says, okay, if something doesn't specifically
 match down here, it matches

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 this. This is called
 the default route.

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 And once again, send
 it to the gateway.

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 Actually, I'm sorry, let
 me take a step back here.

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 When I said 10.7.1.30, all of these entries
 right here, all these entries

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 that say on link, that says I can
 talk to that device directly.

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 So all of these things mean if I have
 a packet that matches any of these

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 entries, I can put it into an ethernet
 frame and I can speak directly

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 to it. I don't need
 to use a router.

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 He is on my network.

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 The only entry here that matches the
 router is this first one, the default

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 gateway. So if my laptop creates a broadcast,
 well, a broadcast is right

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 here. This says match
 all these bits.

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 That means it's on link.

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 That means put that broadcast
 on the wire.

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 If my laptop creates a multicast, well,
 a multicast in binary, 1, 1, 1,

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 0, x, x, x, x. So any IP address where
 the first four bits start out with

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 this, that is a class D address.

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 Class D is in David.

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 And class D addresses were
 reserved for multicast.

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 So these first four bits
 come out to 224.

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 So right here, anything that begins
 with 224 that says put on my wire,

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 it's in my network.

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 But if I gave it, like I said, 888
 or 70.111 or anything else, all of

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 that is going to match this entry right
 here, which says send it to the

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 gateway. It's not on my network.

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 I don't know where it is, but hopefully
 he will know where it is.

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 And once again, the command to view
 all that in DOS was route print.

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 Route print gave us that.

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 So that's just a little overview there
 of how your laptop identifies its

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 routable address, how it extracts from
 the route old address, what network

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 it's on, and how it knows what to do with
 packets that are going to destinations

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 that are not on your network.