[music]
So with what we've been doing so far with
subnetting we've been focused pretty much
exclusively on given some base starting network which
you can't touch, you can't modify, and then asking
yourself, okay, how many networks do I actually need
in my organization beyond this one? Oh, I need eight. I
need 15. I need 37. Figuring out, okay, of my available
host bits how many of those do I need to steal? How many
do I need to convert in to subnetting bits so I can
take this one network I originally got and subdivide
it into more networks to fit the quantity of networks
that I need? The 2nd piece of that is, you know, each one
of those networks in your organization is going to
have a certain quantity of hosts in it, right? If
you're creating a network for your engineering
department, if you're creating a network for your
payroll department you're going to have to also be
aware of, okay, not only is that a discrete network that
needs its own group number, its own network number, but
you also need to be aware of how many devices within that
network are going to need discrete IP addresses, how
many hosts will there be in that network. And also, keep
in mind that whatever router is connected to that
network, whatever default gateway is connected to it,
that router counts as a host. That router's
interface connecting to that network is going to need an
IP address itself so he recognizes that he is a host
on the network that is going to consume one of your
available host addresses. So don't forget about that. So
that's the second part of this, is okay, not only do I
need to create my subnets and figure out what my
subnet mask is, but after I create that new subnet mask,
I have to ask myself, These subnets I'm creating, do
they have enough host space left over for the host
requirements I have in each one of these networks. If
the answer is no, well then you can't use this technique
that we're using here. You have to use different
technique that will get into them. We talked about
variable links subnet masking. So how do you
answer that question? How do you say, Okay, given this
particular network, whether it's my starting network or
whether it's a network I've created as a result of
subnetting, how many host addresses do I have
available in this network? And here's the formula to
answer that. You take-- actually hold on a second.
This is incorrect. This is incorrect. I need to change
this. You're not talking about subnetting bits at
this point. You're talking about host bits. There we
go. That is more accurate. To the power of H minus 2.
So when the determining how many subnets am I going to
have available. You took a look at those bits you converted
into subnet bits. Like I converted 3 bits in a
subnetting bits, and I said, "Well, with 3 bits of
subnetting, 2 to the power of 3 gives me 8 combinations
- 8 networks I could have." Now, you're taking your
gaze, your focus, and you're shifting it away from your
networking bits, and now you're looking at the bits
that are left. You're looking at your host bits
that are leftover, and you're saying, "Okay, how
many host bits do I have?" If I've got 9 host bits
leftover, if each one of those subnets I've created
has 9 host bits leftover at the end, I say, "Well, with
9 host bits, 2 to the power of 9 minus 2 gives me how
many host I have available?" Now you might be wondering,
well why minus 2? Well, the last part of the slide here
demonstrates that because remember, of your host bits 2
of those available patters are not accessible for
assigning to hosts. The all 0s host and the all 1s host, the
network and the broadcast are not usable. So that's
why you don't have every possible combination of hosts
available. You have every possible combination but 2
minus 2, so 2 to the power of H minus 2 gives you your
available hosts. So, based on that, let's do a few more
examples, or I should say, a few more exercises. Get
everything lined up here. All right, so here we go.
Given the following subnet mask, how many hosts can fit
in this subnet? What's the total quantity of host you
have available based on this subnet mask? So in this
particular example, our correct answer is B, 62
hosts. So with this particular subnet mask, it's
telling us that of our entire 32-bit address, most
of it, 26 bits, are networking bits, are
reserved for the network, for the group, which only
leaves us with six bits left over for host bits. Well, 2
to the power of 6 gives us the number 64, and when we
subtract 2 from that, that gives us 62. So another one.
An ISP leases you the following network. You need
to create 22 subnetworks from this single network.
What will be your new subnet mask, and how many hosts
will be supported in each subnet? All right, so the
answers for this are A and E. Good job for those of you
who selected that. So with 22 subnetworks. Okay, so
let's see 1 stealing 1-bit will give me 2 networks, 4
networks, 8, 16, almost there, 32. So I need to
steal 5-bits to give myself at least 22 networks. So if
I take 5-bits and I add it to a slash 23, 23 plus 5 is
a slash 28. And with a slash 28, that will give me the
first subnet mask, 255.255.255, that gives me
24-bits. And then 240, if I draw that on here, that's
1.1, that's 224 plus 1, so that gives me my 28-bits.
First 23 plus the 5 bits that I stole gives me 28
bits, which leaves me with four host bits left over.
Two to the power of four minus two gives you 14
hosts. An ISP leases you the following network. You need
to create 22 subnetworks from this single network.
See if you can answer all four of those questions.
[silence] Okay. I gave you a little bit of extra time on
that one because it's basically four questions
rolled into one. Here are the answers.
Okay, so we start out with the /24 to make 22 networks
out of that. It was just basically like the previous
question, that also has 22 networks. We needed to steal
how many bits, 2, 4, 8, 16. We need to steal 5-bits, 2
to the power 5, gives us 32. So that gave us our new subnet
mask of /29. Which in dotted decimal is this. It
tells us the first 29-bits, our networking bits. With the
/29, that means we have 3 host bits left over. 2 to
the power of 3 is 8 minus 2 is 6 hosts. What is the
subnet address of the 4th subnet? Let me actually show
you I've been holding off on this little tidbit until
now, a little cheat sheet I developed to help identify
this kind of thing. Right here, this is what I call my
subnetting cheat sheet. Everybody's got a different
way of doing this. You'll probably find dozens of
similar types of things if you Google it, but this is
just one I came up with. This is the way I work it.
So in the previous example we figured out it said I
needed 22 subnetworks. So if I use this I say, okay, well
the numbers in green here represent how many networks
are available. So if I need 22 subnetworks that means
I'd have to go out to right here, so that means I would
need a /29 so putting my dividing line right here
gives me 32 networks so /29, and then if I look just to
the right of that line that gives me how many hosts
are available. So basically the way you create this from
scratch is this, and this is really useful if you're just
subnetting into the last octet, into the 4th octet.
You just say, Okay in that 4th octet, let me just put
my numbers in binary here, 1, 2, 4, 8, 16 so on and so
forth, then draw lines separating each bit. And so
for here you can see, Okay, well if I steal 1-bit that's
a slash 25, if I steal 2-bits it's a slash 26. So
that's where you count the blue numbers up here. This
is your shorthand notation for your subnets. And you
can see here, Okay, now I just started doing binary 2,
4, 8, 16, 32. This tells me, Okay, if my-- if I have a
/25 it gives me 2 networks. If I have a /30
it gives me 64 networks. If I have a /27, 8
networks. So now you've developed how many networks
you get for each one of these subnet masks and then
you take a look at your original binary number,
subtract 2 from it, and put it to the right of the line.
Remember, from the right rear most we discover our
hosts. So, that gives me 126. 64 minus 2 is 62. 32
minus 2 is 30. So now once I've decided what my line is
going to be I can look to the left of it to discover
how many networks I have available and I can look to
the right of it to discover how many hosts I have
available given that particular subnet mask. So
in this particular case when we saw that I had-- let's
see, let's go back to here - when I was going to be doing
a /29, right? 255.255.255.240 that's /29.
So right here with the /29 when I looked just to the
right of the line. That gives me six hosts and
indeed that was this correct answer right here, six
hosts. Now one other thing you can use this for was the
subnet address of the fourth subnet. If I go back to
this, once I know where my dividing line is, like for
example, /29, the dividing line is right here between
where the network ends and where the hosts begin. Look
at the binary number just to the left of that line. In
this particular case, it's the number eight. This tells
you that all of your subnets are going to be in multiples
or increments of eight. For example, you're going to
start out with subnet 0, 00000, and then it's going
to be 00001. Well that's number eight. 00010, that's
number 16. 00011, that's the number 24. What do they 179 00:12:58,044 -a subnet mask that was a slash twenty-seven, well just looking to the left of this line, all of your subnets are going to be in multiples of thirty-twos. They're all going to be in multiples of thirty-twos. So in our particular case, where's my cursor, there it is, we came up with 255, 255, 255.248, which was equivalent to a slash twenty-nine. So there's my last octet put into my binary numbers. Nope, hold on a second. I think I'm missing something. I think I only had seven patterns. One, two, three, four, five, six, seven, eight. All right, there we go. So here we've got 128, 64, 32, 16, eight, four, two, and one. Okay, and when I put my dividing line as a slash twenty-nine, which was right here, I've got twenty-four bits plus five slash twenty-nine. Now I know that all of my networks are going to be in increments or multiples of eight. So my first network, see what they say, the original starting network was 199.10.1. 199.10.1. And I can't change that. So the first network is going to be .0, that's my first subnet. And now my next one in multiples of eight is going to be that. My next multiple of eight is going to be that. That's going to be my next multiple of eight there. Let's just do one more. That's my next multiple of eight. And the question was, what is the subnet address of the fourth subnet? Well, we just figured that out. Here's the first, second, third. So this was the subnet address of the fourth subnet right there, 10.1 .24. That's what led to this. What is the broadcast address of the sixth subnet? Okay, well let's keep going. We've already got the fifth one. The sixth one will be that. Let's just go one more to the seventh one. That's the seventh subnet. So if this is the IP address, if 1 .48 is the IP address of the seventh subnet in binary, one less than that would be the broadcast address or the very last address of the previous network. Well, what's one less than that? 199.10.1.47. So that is the broadcast address of the previous or the sixth subnet. And that's what gave us this number. So if you didn't have a chance, let me let you take a screenshot of that for a moment. I'll get rid of this circle here. And using that, let's see how well you fare on this next question. Same type of question. Go ahead and read through that. So let's go through each one of these. So we need 59 subnetworks from the single network. Okay, so let's just use my fingers here. 248, 16, 32, 64. So I'm going to need to steal six bits. So if I take my original 21 bits and I add six bits to it, that gets me a slash 27. So let's go over here. And we'll just put a red circle around slash 27. So that's where we are. So based on that, what will be our new subnet mask? 255, 255, 255.224. That's equivalent to a slash 27. How many hosts will be supported in each subnet? Well, take a look at this. Go down to the bottom right. 30 hosts. That's the number just to the right of that line. 30 hosts. What will be the subnet address of the fourth subnet? Well, looking at this with a slash 27, my subnets are always in increments or multiples of 32s. So the first subnet will be the zero subnet and then the 32 subnet, the 64 subnet, and the fourth subnet will be the 96 subnet. The 96 subnet. What will be the broadcast address of the sixth subnet? Well, if we kept on going adding 32 plus 32 plus 32, if we went up to the seventh subnet, we would have seen that that was the 8.192 network. And if we back off one from that, that gives us .191, which would be the broadcast address of the previous or the sixth subnet. It was right before it.