There's a lot of
videos out there that show you how to
perform IP subnetting, but it usually involves
some type of math. You have to understand
the powers of 2. You need to understand
how to convert from decimal to binary
and back to decimal again. And one of the
challenges that you're going to have when you sit
down for a certification exam is you may not have
the time needed to be able to perform
all of those mathematical calculations. You also have to keep in
mind that you're sitting down for an exam and that process
can be rather stressful. It's difficult to
be able to calculate these things on the fly
as the clock is ticking. So I wanted to put
together this video that showed the process that I use
during a certification exam to be able to subnet
very, very quickly without having to go through
all of those mathematical processes. Well, there's a little
bit of math involved. You do have to be able to use
some simple addition to create some tables prior to sitting
down and starting the exam. And when you perform
the actual subnetting, you have to be able to
subtract one and add. One and during an exam, I
don't think that kind of math is something we need to
worry too much about. This is really a combination of
a lot of different processes. I wanted to create a series
of subnetting processes that got rid of the math and
tried to make the process as simple as possible, removing
all opportunities for you to make a mistake
during an exam. Now, if you're already
familiar with subnetting, some of these processes
may be oversimplified, and that's fine. You can skip over the things
that you already know. But if you're like me and that
stressful test situation is one where it's very
difficult to concentrate, then this seven second
subnetting process is going to be for you. Here's a common
subnetting problem you might be faced
with on an exam. You're given a
network configuration. In this case, we have
four separate networks. You're given an IP
address assignment, 192.168.1.0 with
a /24 subnet mask. It says that we need
an IP addressing scheme with more than one
network address that can support 40 devices per subnet. So we've been given the
single network address. We need to subnet it out
into at least four networks, and those four networks
need to at least have 40 devices per subnet that
can be supported with that IP addressing scheme. If we wanted to write
this out in a longer form, we could, of course,
write down all of the different
subnet mask options. We could convert those
to binary so we can use those for calculations later. We might want to
reference that back to the CIDR block notation. And we'll be able to
see how many networks and how many hosts
per network would be for each individual
subnet mask. Obviously, on an exam you
don't have this kind of time to write out one of
these extensive charts to be able to start calculating
a subnet configuration. To do this a little bit faster,
we might want to use the power of 2's. We can, of course, create
a power of 2 chart, or memorize these powers of 2. And then we would look at
our subnet mask, so /24. So we have 24 bits that
are already masked. So we might want to borrow
some bits from the ones that are left over. We might want to borrow
2 bits for a subnet mask, and then use 6 host bits
to use on top of that. Now, we know that with those
2 bits that we're borrowing, those subnet bits we
would calculate as 2 to the second power, since
we're borrowing 2 bits there. And that would give
us a value of 4. So the total number
of subnets would be 4. We've got 6 host bits, so
it'd be 2 to the sixth power. And calculating that
out would be a 64. And of course, we
subtract 2 from that to subtract the broadcast
address and the network address. And that leaves us with 62 as
the number of hosts per subnet. And there's the answer
we were looking for. We now have 4 subnets and
we have 62 possible hosts per subnet, all because
we were able to remember that powers of 2. But of course, even this
process requires that you do a little bit of work. You need to convert
things to binary. You need to figure out how many
bits you're going to borrow. You need to calculate this
based on the powers of 2. And you either have to have
a table already prepared or you need to remember
those powers of 2's so that you can use them very,
very quickly on an exam. Remember that when
you're subnetting, you're really looking for
four different addresses. The first one is going to
be your network address, some people call this
the subnet address, and it's the very first address
that is in a particular subnet. We also want to find
the broadcast address. That is the last
address in the subnet and it's the one that is
used when a broadcast is sent on an IP version 4 network. We also need to know
the first available host address on the network. This will be the one that
our workstations and devices will use on that network. And obviously, we
then need to know what the last available
host address will be for that network so that we
can determine the exact range to use for all of our devices. Well, if we take our
previous example, you can then see that we
would have to write out all of these in binary. We would have to determine the
different subnets by changing the binary from 00 to
01 to 10 and then 11, and then calculate all of
the first host and last host addresses by comparing the
differences in the binary settings for all of these
different addresses. Obviously, on an exam
you don't have time to go through this
entire process to create all of
this information. That's why I created the
seven second subnetting process so that we could very
quickly determine all of these in just a matter of seconds
and then continue on with what we needed to do for the exam. If you've looked at
other subnetting videos or you've perform some
subnetting yourself, you'll notice there are
certain patterns in the math. For example, a 255.255.255.0
subnet mask is a single subnet and it's a range
between 0 and 255. If we grab 1 extra
bit off the end and make it 255.255.255.128, it
effectively cuts that in half. So we have a range
between 0 and 127, and another range
between 128 and 255. If we borrow another bit and
make it adopt 192 at the end, you'll notice we cut it up
again into smaller pieces where you're separating these
in half every single time. So now we have a process
that we can easily see is occurring
every single time, and we are going to be
able to take advantage of this very common
theme whenever we're trying to subnet. We're going to create some
tables, and from those tables we'll be able to calculate
exactly what the subnet might be without having to go
through any of the binary math. We're going to make two charts
to help with this subnetting process. The first chart
is going to be one that will help us convert
from a CIDR block notation to a dotted decimal notation. This is especially
helpful when you're given one of these
questions that will require you to convert,
and having this chart available will allow you to do
that very quickly. The way I like to
lay this out is to make four separate
columns, and each one is associated with one of
the octets of the IP version 4 IP address. I put a /1 through /8
in the first column, /9 through /16 in the
second, /17 through /24, and /25 through /32
in the fourth column. Then I want to calculate
how many networks will be available in each one of these. And I put a 2 on the
first row, and then I multiply that by 2 each time. 2 times 2 is 4. 4 times 2 is 8. 8 times 2 is 16. Until I get to the bottom and
the last number should be 256. Then I create an
Addresses column, and I usually start at the
bottom with the number 1. And again, I'm
doubling each time. I double to 2, 2
times 2 is 4, 4 times 2 is 8, until I get
to 128 addresses. And now the final column,
which is going to help me make that conversion between
the CIDR block notation and the decimal notation. I'll start with 128 at the
top, and then I'll add in 64. So I'm effectively adding as I
go down the Addresses column. 128 plus 64 is 192. 192 plus 32 is 224. 224 plus 16 is 240. Until you get to
the bottom with 255. The second chart I
create is one that shows me the address groupings
given a particular subnet mask. If the subnet mask has a
128 as that subnet mask, then I know that the subnet
will be split into two. There will be a
starting number of 0 and a starting number of 128. If it's a 64 as the number
of hosts in a subnet, then I know that my
subnet boundaries will be 0, 64, 128, and 192. And I write all of these out. It's obviously pretty easy to
memorize an address boundary at 128 addresses, or
64 addresses, or 32. But when you get into
these smaller numbers, especially as it gets
higher up in the list, it becomes a lot more
difficult to remember where the exact boundary is. And remember, we're trying to
do this as quickly as possible. We don't have time to calculate
all of these on the fly. So I write everything
out during this period so that it's now sitting
right in front of me and I can perform any
type of subnetting now in seven seconds. Of course, all of these charts
are the ones that work for me. You may find a different
layout or a different way of performing this might
work a little bit better. The key for me is
to get everything on the page, that
way I don't have to perform any of
these calculations during the exam itself. Now that we have our
charts available, we're able to subnet. The things that
we're going to be able to derive from
these two charts is that we're going
to be able to convert the IP address and a CIDR block
notation into the decimal form. That first chart that
we created allows us to do this very quickly. That chart is also going to show
us how many devices per subnet will be on this
particular network. The second step of
the subnetting phase is to calculate the
network address. Some people call this
the subnet address. And that second
chart that we created allows us to very quickly
find the beginning and the end of that particular subnet. Thirdly, we need to know
the broadcast address. This will be the last address. And again, that
second chart shows us what that ending boundary is. And lastly, we need to
calculate the first usable and the last usable IP
address of the subnet. That's very easily
done by simply adding 1 to the network address
and subtracting 1 from the broadcast address. Lets try doing a
few of these and see if we can get the hang of it. The first subnetting that
we're going to do is address 165.245.12.88 with
a /24 subnet mask. Generally, the subnet masks that
end on the 8 bit boundaries, the /8, the /16, and the /24 are
generally something that's easy to calculate. But let's go
through this process to show you how it would
flow, and then we'll try some harder subnet
masks after this. The first thing we need to do
is convert both the IP address and the subnet mask
into a decimal format. The IP address is already
in decimal format, but this /24 needs
to be converted. If we look at our first chart,
we need to find the /24. It's in this third column. That's important for later. And you'll see that
it converts to a 255. So let's write these down. We've got our IP address
at 165.245.12.88, and my subnet mask has a 255 all
the way into the third column. So we're going to use
255's all the way up until we hit that third column. And since we are in
that third column, everything after that point will
have zeros as the subnet mask. I'll show you in a
later example how this changes if the mask
is not a 255 or a 0. Now that we have both
our address and our mask, we need to calculate
the network address. If the mask is
255, you bring down the address value at the top. And if the mask is
0, you simply use a 0 in for the network address. That means that if we bring down
the 165, the 245, and the 12, and then bring down a
0 where the mask is 0, our network address
165.245.12.0. Now we need to calculate
the broadcast address. For the broadcast address,
if the mask is 255 we bring down the address number. Exactly the same
process as if we were calculating the subnet address. But if the mask is 0,
we make that value a 255 instead of a 0. So that means our broadcast
address is 165.245.12.255. Now, the process of
determining the first usable IP address and the last usable IP
address is simply adding a 1 and subtracting a 1. So the first usable IP is
the network address plus 1. So with 165.245.12,
plus 1 to that 0 is a 1. And the last IP has
a broadcast address minus 1, which means
it's 165.245.12.254. And you can see
they're listed there. So for this example, you can
see what the network broadcast first IP and last IP addresses
would be for this particular IP address and subnet mask. Now let's try a
subnetting problem that's a little more
difficult. This one falls into the fourth octet. It's the same IP
address, 165.245.12.88, but on this network this
particular device is on a subnet /26. Let's perform exactly
the same process and see how quickly we
can calculate the results. The first step is to convert
the IP address and the mask to a decimal. We know we have the IP
address of 165.245.12.88, but we've got this /26. So we need to use our first
chart to find the /26. And you can see that the /26
is in this fourth column. That means that
whatever value we find here, which
happens to be 192, is what we're going to
put into the fourth column of our subnet mask. Everything prior to this
number will be 255's. And if there were open
octets after this, those octets will be 0. So for our subnet mask, since
this is in the fourth octet, it's 255.255.255.192 is
that conversion for the /26. Another thing to notice as we're
determining what the subnet mask is, it was a /26 that
converted to a decimal 192, but notice that the number
of addresses is listed as 64. That means we need to go
down to our other chart where we have listed out
all the addresses as 64 and determine where that
number happens to sit on that particular column. That column was
the fourth column. That was the one that was
different than a 255 or a 0. And an 88 is the
number in the address. So we need to find in
this particular row where an 88 would fall. And in this particular case,
it falls into this range that begins with a 64. That will be very
helpful when we start determining where the
beginning of the subnet is and where the end
of the subnet is. So let's do that. Let's calculate the
network address. If the mask is 255, we
bring down the address value into a column. If the mask is 0, we'll use a 0. We don't have any zeros in
our mask in this example. For any other number,
we refer to the chart. And again, we had
a 192 there, so we need to refer to our
chart on that 64 row and determine where the 88 sits. And we know that that
particular range starts at a 64. So our network address is
going to be 165.245.12.64. Our next step is to find
the broadcast address. And we've already done
the work up to this point. We really don't even have
to calculate anything because we know exactly
where in this chart this particular range
is for this subnet. We know that the
network address is 64. So at the other end of this
range is the number 127. We know it's 127 because the
next range, or the next subnet, starts at 128. So very easily we can then
bring down 165.245.12.127 and we have our
broadcast address. And just like our
previous example, to find the first usable IP
address and the last usable IP address we add 1
to the network address and we subtract 1 from
the broadcast address. That means that the first
IP will be 165.245.12.65, and the last usable IP
will be 165.245.12.126. And there's the answer for
the subnet 165.245.12.88/26. Let's run through
another example. We'll use exactly
the same IP address, but notice the subnet mask is
different again, 165.245.12.88. This particular IP address
is on a subnet /20. Same process that
we used before. We need to convert
that /20 a decimal. So we're going to bring
down the address value. We're going to find
the /20 in our chart, and when we find that row we'll
see that it's associated with 240. But you'll notice that the
/20 is in the third column of our chart. That means that the 240 is going
to fall into the third column of the subnet mask. Everything before
that value is 255, everything after
that value is a 0. We also know from our
chart that there are 16 addresses in this subnet. So if we look at our row where
all of the 16 delineations are, we need to find that number
for that third column. Everything focuses on the column
where that value is a little bit different. So that 12 that is in the
address needs to be put into this row where
there's all the 16's. Now, 12 is certainly
between 0 and 15, so it's in that first section
of that particular row. So we know now what
the exact range is going to be for
the network address and for the broadcast address
by very quickly determining where it happens to sit. So we want to be
able to bring down any of the network addresses
where the mask is 255. If the mask is 0, we use a 0. But for that column
that has that 12, we know that it's actually
going to start this range at 0. So our network address is
going to be 165.245.0.0. The broadcast address
works exactly the same way as the other examples as well. If there's a 255 we
bring down the address. If there is a 0, we
bring down a 255. But again, we've got
this third column. So we need to look to see what
our range is for this column. And because it is the 16's,
we know that it goes from a 0 to a 15. So we can look at
our chart and know immediately it's 165.245.15.255
is our broadcast address. From here, of course, to
calculate the first usable IP and the last usable
IP, we're simply adding 1 and
subtracting 1, which makes our first IP
165.245.0.1 and the last IP is 165.245.15.254. You can start to
see how fast you may be able to make this
work once you become accustomed to knowing
where to look in the charts and where to find the
information that you're looking for. Let's try one last example. Let's try an address
18.172.200.77/11. It's the same process
we used before. We're going to convert
that /11 to a decimal. So we need to find
the 11 in our chart. It's in the second column. You can see the /11 is a 224
with 32 addresses per subnet. So if we write this
in, it's 255.244 again. We put it in the second column
because that's where we found the /11. Everything prior to
that column is 255. Everything after
that column is a 0. Now that we have the address
and the subnet mask written in decimal, let's find
the network address. We know that we're
going to be using these 32 addresses per subnet. We're going to look at
our row where the 32 is and we're going to look
at our second column. That's where we found the /11. The address values 172. So we need to find in our row
where the 172 happens to be. And it's in this range
right here, between 160 and before the next
subnet starts at 192. So it'll be 160 through 191. Well, now we already
have the values we need to calculate the network
address and the broadcast address. So we'll use those numbers. With the 160 we'll
bring down the 18. We'll use the 160. And of course,
where there's is a 0 we bring down the 0 for
the network address. For the subnet address, we
use the last value that's in this range, which is a 191. So it would be 18.191. And of course, where there are
zeros we bring down the 255 and that's our
broadcast address. And for the last
step where we want to find the first usable IP
address and the last usable IP address, we'll add 1
to the network address and subtract 1 from
the broadcast address. That makes our first usable IP
18.160.0.1 and our last IP is 18.191.255.254. And there's the answer for the
IP address subnetting for this IP address of 18.172.200.77/11. Now that you
understand the process that I use to understand
this during an exam, you can of course
try this yourself. Write down your own
charts, customize them to fit best for
the way that you work, and try some IP address
problems yourself. You may find that you're able
to perform these very quickly, in a very short period of time. And it's certainly a good way
to check your work when you're in the middle of an exam.
