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[music]

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So let's take a look at our

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whiteboard for a moment.

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We've got this concept here of

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let's just say 2 hosts

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running IPv4.

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Let's just put 3 hosts on here.

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Say 2 laptops and a router.

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We're going to put them all 3 on

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the same exact Ethernet broadcast

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domain, the same Ethernet segment.

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Okay, so let's give some IPv4

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number to these 3 hosts.

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Now, we know that because these

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3 hosts are on the exact same

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network domain part of their

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IP address and binary

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from left to right

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has to represent the group or the

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network they're a part of.

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So let's say I give them this,

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actually I have to make this pretty

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small here to fit this in.

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So, let's say this guy here is--

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and I'm going to go ahead and

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expand this so you can see

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a little bit better.

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So 1, 0, 1, 2, 3, 4,

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5, 6, 7, 8, 9, 10,

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11, 12, 13, 14, 15,

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16, 17, 18, 19, 20,

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21, 22, 23, 24, 25, 26, 27, 28.

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All right, I'm not going to count

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all up but let's just assume that

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that's 32-bits.

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I think I'm pretty close there.

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And then we'll put one more

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address on this guy right here.

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I'll do that up at the top

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so we have room.

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Okay, so in binary these are the

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32-bit IP address I've given to

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each of these hosts.

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Now, if we were looking at these

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addresses from the original

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implementation of IP back

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 in the late 1960s, early 1970s,

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the IP code or software that was

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running on all 3 of these devices

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would interpret these addresses

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the exact same way.

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It would say the first 8 bits

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represent our network.

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That's not 8 bits right there.

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1, 2, 3, 4, 5, 6, 7, 8.

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There we go.

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1, 2, 3, 4, 5, 6, 7, 8.

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So the original implementation

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of IP would have said this is

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the name of our broadcast domain,

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10010010.

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That is our network.

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All of these other bits

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are unique patterns,

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host bits, for each of our hosts.

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Now I said here that the problem

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with this is if I take 8 bits,

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that's the very first pattern

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I can come up with.

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Well we know that's reserved.

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No network can be the

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all 0s network.

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Then we have a whole bunch

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of combinations.

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The last pattern in binary

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I can come up with is that.

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We know that's also reserved.

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So everything in between here

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is 254 combinations.

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So if I'm limiting myself to the

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original implementation,

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here is one combination

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I've just used, 10010010

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which leaves me 253 networks

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which are remaining,

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and that's not very many networks,

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not very many broadcast domains.

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Now with classful,

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if I go into the operating system

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on all 3 of these devices,

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and I change the IP code

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in a little bit and I say,

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okay, you are now classful devices,

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look at your IP address in a

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classful way.

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Now instead of just assuming

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that the first 8 bits

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represent the network,

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they don't make that assumption.

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See if I get rid of this box here

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if it'll let me. There we go.

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Now instead they do something

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else first.

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All these 3 devices independently

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take a look at the first few bits,

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and as classful devices

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they say, oh okay, well the first

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2 bits that we have are 10,

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so that tells me that now my

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network is the first half

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of this address,

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the first 16 bits.

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So 1234, 1234, 1, 2, 3, 4.

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1, 2, 3, 4. Right there.

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So that is the network address,

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the first 16 bits.

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Now, how does that give

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us more networks?

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Well let's focus here just for a

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moment on the first 8 bits because

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remember we, as humans,

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read this address in

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4 dotted decimal numbers.

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So what is the first byte?

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1, 2, 3, 4. 1, 2, 3, 4.

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So that's the first byte

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right there.

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Let's go ahead and translate that,

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first of all,

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to a dotted decimal number.

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So we've got the 128-bit,

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the 16-bit, which is 144 plus 2,

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so this is the 146 network if I did

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my math correctly.

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Now, in the original days where the

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networks was just the first byte,

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the first 8 bits,

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that means only this broadcast

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domain could start with 146.

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There were no other networks

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available out there that could

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start with 146.

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This number was reserved

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for this wire,

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this broadcast domain.

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But now with classful addressing,

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now that we're saying the first

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2 bytes represent the network,

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that means one wire could be

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146.0.anything.

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We could have another wire,

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another broadcast domain

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that was 146.1.anything.

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Another wire that's 146.2.anything,

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on and on and on,

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all the way up to 146.255.anything.

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So that means just for this

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single number of 146

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we have 256 combinations

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of networks.

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This one right here -

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I'm not sure what this is - this

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is 146.-- let's see here,

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let's go ahead and add up

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the second octet.

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This is 192 plus 8, that's 204.

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So everybody on this particular

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wire recognizes that their group

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number - their network number -

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is 146.204.

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So now with classful addressing,

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because I'm able to look at more

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bits of this 32-bit address,

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that gives me a

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lot more combinations.

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So with Class B right here

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- we know that Class B

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starts out with 128.

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Well, just with 128 I could have

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a wire that's 128.0,

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another wire that's 128.1.

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I've got 256 combinations of 128,

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another 256 combinations of

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networks that begin with 129.

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Another 256 combinations of 130,

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all the way up to 256 combinations

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of 191.

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That's what gives us

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in the Class B space,

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over 65,000 unique networks

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because we have so many

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more bits available to us.

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And for this to work,

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the system that it's running on

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the laptop, the PC, the mainframe

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has to be able to look at this

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address in a classful way.

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The code has to tell it,

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the IP code,

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the software developer has

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to write the code in

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such a way that says, look,

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don't make any assumptions

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about your address, instead,

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look at the first few bits.

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And whatever the bit pattern

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is in the first few bits,

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that will tell you of

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your 32 bit number,

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how much of it belongs

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to your group, your network,

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your broadcast domain.

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And that's what gave us thousands

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and thousands and thousands of

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more networks that were available

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than when all we had was just our

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first byte and we couldn't go

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beyond our first byte.

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So that's why classful addresses

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gave us that ability to scale

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beyond a much larger

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addressing space.

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[music]