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[music]

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Just to review, we know
what a Subnet mask is. It's

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a comparison tool. Now for
us as humans, it's nice when

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we see an IP address and a
Subnet mask, and the Subnet

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mask falls in a nice
boundary of a byte like

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255.000 or 25525500. Right
that's real easy for us

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using our eyeballs to do
that comparison and say,

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"Oh, okay clearly this octet
and this octet belong to the

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network. Everything else is
host bits." Well I've been

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trying to show you with some
of these quiz questions here

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is the Subnet mask gives you
the ability to draw that

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dividing line anywhere in
that 32-bit IP address. It

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doesn't have to fall right
at the end of a byte. So,

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for example, in this
particular case it does fall

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right at the end of a byte.
Right? Right after the

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second byte, our subnet mask
ends. And so it says, Okay,

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the first two bytes with the
/16 belong to the network.

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Another one where it falls
right at the end. So in this

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case, with the /24, that
says the first three entire

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bytes belong to the network.
No problem, but you could

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easily do it like this, /28
where you say, Okay, the

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subnet mask extends all the
way into the last octet.

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Four bits of the last octet
belong to the group. So in

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that last example, example
number three there, we've

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only got four bits left for
host. So let me ask you some

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questions about what you see
right here. Question number

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one, what was the original
classful network before any

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subnet masks were applied?
So forget about the /16,

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/24, and /28. If we we're
looking at this purely from

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a classful network
perspective, what would the

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classful network have been?
Go ahead and someone type

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that into the question feed
there and answer that for

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me. Does anybody know what
the original classful

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network was? Well, the
original classful network

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was the 1 Network, right?
With the first bit being a

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0, that was a Class A, so we
only looked at the first

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byte. The first byte, if we
translate that back to

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decimals, the number 1. So
that was the 1.0.0 network.

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Next question. In all three
of these examples, what is

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the network address now that
we've got /16, /24, and /28?

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So in this first example
here, example number one,

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given the subnet mask -
we're no longer doing

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classful - what's the new
network address based on

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what you see right here?
Well hopefully, it's pretty

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easy. You just count up the
red bits. The first byte is

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1. The second byte is the
number three, so this is the 1.3

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network. And the second
example, our network address

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in now 1.3.192. And the
third example, the network

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address is 1.3.192.112. That
one's a little bit tougher.

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112 is the fourth octet. Of
these three examples here

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with these three different
subnet masks, which one

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provides for the greatest
quantity of hosts?

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Well hopefully, you very quickly
said example number one

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because example number one's
got a whole lot of host

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bits. The other two examples
don't have as many. Last

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question about this. If this
company was originally given

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the Class A network of
1.0.0.0, which of these

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subnet masks would provide
for the greatest quantity of

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subnets? In other words,
which of these subnet masks

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would allow us to take that
original network of 1.0.0.0

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and carve it up into the
most quantity of subnets

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that we could? Give us the
most variation. Well hopefully

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you said example number
three. With example number

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three, I can't touch my
original network of 1, but

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with a /28, that means I've
got 8, 16, 20 bits of

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subnetting. That gives me 2
to the power of 20

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combinations I can come up
with. Tens of thousands of--

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hundreds of thousands of
networks I can come up with.

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So why do we subnet? So we
can divide a single

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allocated network into
multiple subnetworks. That's

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the whole reason behind it.
Minor loss of host space

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available. So here's a
practical example. Imagine

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if you lease a network from
your ISP for $100 a month.

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The building here we see
requires four networks. If

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we used classful addressing,
that right there would cost

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us $400 a month because we
had to purchase or lease

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four unique networks. With
subnetting, I can take just

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one classful network, subnet
it, I'm only paying $100 a

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month. I'm only paying one
fee for my 20.0.0.0 for my

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ISP, but by extending those
bits out with a subnet mask,

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I have now given myself four
unique networks but I'm only

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paying for one of them. One
of the key points I want you

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to take away from this is
that, as I take my original

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network and I say, Okay, I'm now
gonna take some of my host bits

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and add them to that network
- give me a little bit of

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room to play and have
variation. Well, the more

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host bits you steal-- you'll
see that term a lot,

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stealing bits, which means
you're taking your host bits

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and you're stealing them for
networking bits. You're

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converting them into subnet
bits. The more subnet bits I

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have, there's pros and cons
to that, right? The more subnet

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bits I have available to me,
the more networks I can come

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up with. But, the more
subnet bits I have, the less

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host bits I have. And the
fewer host bits I have, that

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means that each subnet can't
have as many hosts anymore.

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The subnets are getting
smaller and smaller. So

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here's an example of that.
So I got this class B from

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my ISP, 180.1. That's the
network. We know that those

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original 16 bits are
untouchable. Can't touch

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those bits. So we ask
ourselves, how many host

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bits do I have? Well, in
this case, I've got 16.

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Sixteen bits I can play with
on the back end of this

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address. For every host bit
that you convert into a

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subnet bit, you've just
gained two additional

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networks. Two additional
networks. For example, if

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this is what I was given
from my service provider -

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let's say 1.0.2.3.4.5.6.7.8.
1,2,3,4,5,6,7,8. So that's

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what I'm given from my ISP
and I can't touch that.

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Okay, there's my remaining 16
bits of host bits. If I say,

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Well, here's what I'm going
to do. I'm going to take

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this bit right here and add
it to my networking bits.

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I'm going to say, Well, my
group, my broadcast domain

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is the first 17 bits. Well I
know I can't touch this original

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pattern of 16. That was
purchased from my ISP. But

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what can I do? I can say,
Well, I've got one network,

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that's 129.1 and then put a
0 right here. My only other

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option is to put a 1 right
there. I've just now taken

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the one network I got from
my ISP and subdivided into

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two portions. One broadcast
domain is a 17 bit group

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number ending with 0. The
other broadcast domain is a

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17 bit number ending with a
1. You can see, if I move

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it-- let's say I move that
line. I say, Well, I'm

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actually going to now take
two of my host bits. Well,

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now what can I have? Well, I
can have one broadcast

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domain that ends with 0.0.
Another one that ends with

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0.1. Another one that ends
with 1.0. Another one that

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ends with 1.1. So with two
bits, I'm now just giving

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myself four combinations. So
what you're going to see

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here is just basically boils
down to this nice formula. 2

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to the power of subnetting
or 2 to the power of N, gives

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you your quantity of
subnets. In other words--

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let's go back to my example
here. I'm given this

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original network of 16 bits
and I say, Well, hmm, I need

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let's say eight networks.
Well if I use that formula

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to 2 to the power of stolen
bits, or 2 to the power of

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subnetting bits, is it going
to be greater than or equal

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to the number of networks I
need? So now I solve it. I

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say, 2 to the power of SN
subnetting bits is going to

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be greater than or equal to
8, because I need 8

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networks. So you say, "Oh,
well, okay that would be the

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number 3." If I have 2 to
the power of 3, if I steal 3

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bits, that will actually
give me 8 combinations of

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numbers: 000, 001, 010, 011,
all the way up to 111 - 8

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possible combinations. So
you might get questions like

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that on your CCNA or even in
real life, if you're given

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some based network, and
you're told, "Okay, well you

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need 15 networks, or you
need 30 networks, how many

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host bits do you need to
re-purpose or re-convert in a

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subnetting bits to give you
enough variation for the 15

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networks you need, or the 20
networks that you need?"

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Based on that, let's do
some-- oh, before we do

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another quiz, one other key
thing here - one other key

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thing. Let's go back to this
for a moment. In my example,

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I said, "Well, I need to
have 8 networks." You say,

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"Oh, well-- okay, well, 2 to
the power of 3 works, but

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why don't I say, oh I don't
know, why don't I steal this

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many? That looks good. Why
don't I steal 11 bits? Right?

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Because 2 to the power of 11
is going to give me far more

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that 8." "Yeah, but it'll
give me the 8 you need,

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right? Why not just use 8
networking bits? Subnet

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bits?" Well, here's the
thing. Remember, the further

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you push this line to the
right, the more bits you

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steal, the less host space
you have available. If I put

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my line way out here, my
broadcast domain is not

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going to be able to have
very many laptops and PCs

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and printers in it, because
I don't have very many

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addresses left for host
addresses. So when you're

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doing subnetting in this way
that we're doing right here,

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the idea is push your line
to the right, not as far as

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you can go but as minimal as
you can go. In other words,

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you say, Okay, if I push it
just one, will that be

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enough? Ugh, it's not enough. If
I push it two to the right,

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is that enough? Ugh, it's
not enough. Well, if I push

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it three to the right,
that's just barely going to

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give me what I need. That's
where you stop, where the

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number you come up with here
is just barely what you're

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looking for but it just fits
what you're looking for.

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Sure, you could keep pushing
it to the right if you wanted

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to, but now you're sacrificing
host space. So let's do some

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examples of this. So you
were leased the following

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network from your ISP, the
45 network. If you convert

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the first three host bits
into subnetting bits, how

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many total subnets will you
have available? Okay, time's

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up. If we use our formula of
2 to the power of subnetting

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bits gives us our total
quantity of subnets, well,

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if we steal three bits, that's
2 to the power of 3, which gives

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us the number 8. Robert, I see
your question there.

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When stealing, start
stealing point, is it

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connected to the classful
network? So the question is,

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where do you start stealing
the bits from? You have to

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ask yourself, Okay, what was
I originally given? What was

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I given from my service
provider or from my regional

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Internet registry? You know, of
my 32-bit number that they gave

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me, what amount of those
bits do they consider

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untouchable as the
networking portion? Now they

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may have given you a
classful network, or they

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may not. For example, your
regional Internet registry

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or your service provider
could say, Okay, we're

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giving you this,
140.55.64.0/18. So what that

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means is, from the service
provider's perspective, I am

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not allowed to touch the
first 18 bits. That is not a

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classful mask, right? A classful
network would be a /16

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because 140 is a class B
address. But here, my

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service provider has given
me a /18. So that means if

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I'm going to take that one
network and subdivide it

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into subnets, I may have to
go beyond /18. So in this

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particular case of a /18,
let me ask you this. If

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they're giving me a /18, how
many host bits do I have

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available from this one
network that I just

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purchased from my service
provider? How many host bits

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are left? I've got 14 host
bits left, right? I've got

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14 host bits because my
service provider is saying,

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Look, as far as we're
concerned, the first eight

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bits here are networking
bits, the first eight bits

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here are networking bits,
and the first two bits here

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are networking bits. So that
means of the third octet

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I've got six host bits left,
and then of the fourth octet

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00:16:02,829 --> 00:16:07,466
I've got eight host bits
left. So I've got 14 host

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00:16:07,466 --> 00:16:15,508
bits available. One, two,
three. So what this is

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00:16:15,508 --> 00:16:18,578
showing me in this
particular case is, one,

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00:16:18,578 --> 00:16:20,513
two, three, four, five, six,
seven, eight. One, two,

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00:16:20,513 --> 00:16:23,649
three, four, five, six,
seven, eight. One, two. So I

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00:16:23,649 --> 00:16:27,620
can't touch those. So this
is what they've given me

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00:16:27,620 --> 00:16:37,029
right here, so I'll just say
this is untouchable. So now,

225
00:16:37,029 --> 00:16:42,401
what's left over? One, two,
three, four, five, six. One,

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00:16:42,401 --> 00:16:45,438
two, three, four, five, six,
seven, eight. So now I'm

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00:16:45,438 --> 00:16:47,940
going to start stealing from
right here. If I'm going to

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00:16:47,940 --> 00:16:51,277
start creating my subnets, I
have to start from the right

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00:16:51,277 --> 00:16:57,450
of this line. So my subnets
will be a /19, /20, /21, but

230
00:16:57,450 --> 00:17:00,486
I can't go anything less
than /18 because /18 is what

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00:17:00,486 --> 00:17:04,957
they gave me. And yes,
Robert, that number that you

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00:17:04,957 --> 00:17:08,561
just did that this right
here, I'll just put it right

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00:17:08,561 --> 00:17:10,496
there. That's how you
determine your host

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00:17:10,496 --> 00:17:15,434
requirements. That's
actually the next slide. So

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00:17:15,434 --> 00:17:18,104
right now we've just been
focusing in on how many bits

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00:17:18,104 --> 00:17:21,140
do I need to steal to come
up with a number of subnets

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00:17:21,140 --> 00:17:29,181
that I need. Here's another
one. You are leased this

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00:17:29,181 --> 00:17:32,451
network from your service
provider. If you convert the

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00:17:32,451 --> 00:17:35,855
first six host bits into
subnetting bits, how many

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00:17:35,855 --> 00:17:40,359
total subnets will you have
available? If we use that

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00:17:40,359 --> 00:17:49,135
same formula, 2 to the power
of 6 gives me 64 subnets

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00:17:49,135 --> 00:17:55,007
available. That's right, 2
to the power of 6. Now let's

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00:17:55,007 --> 00:17:57,443
take it up a notch, make it
a little bit more

244
00:17:57,443 --> 00:18:01,747
complicated. You are leased
the following network from

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00:18:01,747 --> 00:18:06,686
your service provider. If
you convert the first three

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00:18:06,686 --> 00:18:10,890
host bits into subnetting
bits, what will be the

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00:18:10,890 --> 00:18:18,230
subnet address of the fourth
subnet? Some of you may have

248
00:18:18,230 --> 00:18:20,266
been able to work this one
through, some of you didn't.

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00:18:20,266 --> 00:18:25,371
Let's go ahead and look at
the logic behind this. Okay,

250
00:18:25,371 --> 00:18:27,907
so we're starting with 45. I
cannot touch those first

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00:18:27,907 --> 00:18:31,444
eight bits, and we're saying
I'm going to be subnetting

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00:18:31,444 --> 00:18:34,614
my next three bits. So that
gives me these various

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00:18:34,614 --> 00:18:38,584
combinations. The fourth
subnet? Well, the first one

254
00:18:38,584 --> 00:18:42,154
is 000 and the next one is
001, so the fourth

255
00:18:42,154 --> 00:18:48,227
combination I can come up
with is 011. So if I take

256
00:18:48,227 --> 00:18:52,498
all of my remaining host
bits and convert them into

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00:18:52,498 --> 00:19:00,740
0s, I get that, and so my
fourth subnet is the

258
00:19:00,740 --> 00:19:09,582
45.96.00 network with the
slash 11 mask. Start out as

259
00:19:09,582 --> 00:19:14,120
slash 8 and then I added 3
bits to it making into a

260
00:19:14,120 --> 00:19:19,892
slash 11, so another one of
these. You release the

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00:19:19,892 --> 00:19:24,997
following network,
199.000/24. From the single

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00:19:24,997 --> 00:19:29,602
network you need to create 7
subnets. What will be your

263
00:19:29,602 --> 00:19:38,210
new subnet mask? And the
correct answer here was

264
00:19:38,210 --> 00:19:48,154
255.255.255.2.24. Why is
that? Well we need to create

265
00:19:48,154 --> 00:19:55,294
7 subnets. So we know 2 to
the power of our subnetting

266
00:19:55,294 --> 00:20:04,303
bits must be greater than or
equal or 7. So if we do 2 to

267
00:20:04,303 --> 00:20:10,443
the power of 3 that gives us
eight subnets, so three is

268
00:20:10,443 --> 00:20:16,515
the minimum we can steal. So
if we start out with a /24

269
00:20:16,515 --> 00:20:22,321
and we add three more bits
to that, that gives us a

270
00:20:22,321 --> 00:20:30,196
/27. /27 means all eight
bits of the first octet, all

271
00:20:30,196 --> 00:20:33,632
eight bits of the second,
all eight bits of the third,

272
00:20:33,632 --> 00:20:41,974
and three bits, 111, three
bits of the fourth octet are

273
00:20:41,974 --> 00:20:48,314
subnetting bits. That
would be our new subnet

274
00:20:48,314 --> 00:21:03,229
mask. Let's do one more, and
then we'll take a break.

275
00:21:03,229 --> 00:21:07,767
You're leased the following
network, 145.10 /19. Don't

276
00:21:07,767 --> 00:21:10,369
let that throw you. Just
remember, the first 19 bits

277
00:21:10,369 --> 00:21:13,372
are untouchable. That's how
you need to look at that.

278
00:21:13,372 --> 00:21:18,043
From the single network, you
need to create 58 subnets.

279
00:21:18,043 --> 00:21:28,254
What will be your new subnet
mask? Time's up. There is

280
00:21:28,254 --> 00:21:30,756
your new subnet mask. So how
did you come up with that?

281
00:21:30,756 --> 00:21:33,592
How did we come up with
that? So here's how we do

282
00:21:33,592 --> 00:21:36,929
this. Well, 58 subnets.
Let's see, 2 to the power of

283
00:21:36,929 --> 00:21:46,472
1 is 2, 4, 8, 16, 32 -
almost there - 64. So I need

284
00:21:46,472 --> 00:21:53,846
to steal six bits. That'll
give me up to 64 subnets. So

285
00:21:53,846 --> 00:22:02,521
I need to add six bits to
this. So 19 plus 6 is going

286
00:22:02,521 --> 00:22:13,199
to be 25 or a /25. So that
means here is my 24, and 128

287
00:22:13,199 --> 00:22:17,873
gives me my 25th bit. So
that's how you got that.

288
00:22:17,898 --> 00:22:22,960
[music]