Many people are intimidated
over the IP subnetting part of the Network+ exam. But now that you know
about binary math, you'll find that the process
is relatively straightforward. And you should very easily
be able to calculate all of these IP subnet values. Let's look at an IPv4 address. We've become very
accustomed to seeing these. So I've put one IP address here
at the top-- 192.168.1.131. And just underneath this is the
binary representation of that. You can see each one of these
sections is an eight bit block. We often refer to this as
a byte or even an octet. That means that an IP version
4 address is 32 bits long, or four bytes in length. An IP version 4
address in each one of these blocks, because
it is eight bits-- that means that the
values you can have here would range
between 0 and 255. None of these numbers
can exceed 255 because we're dealing with
eight bits in this single block. Let's now break down what
an IP address really is. If you recall from
our previous video, IP address consists
of two parts. There is a network
address and a host address associated with this. And to be able to break
those two things apart, you not only need
the IP address, but you also need
the subnet mask. So if we take this
example of 192.168.1.0-- that's our network address--
and the subnet mask that has been assigned to this
network is 255.255.255.0. So the first thing
we'll do is write out both the network address and
the subnet mask in binary. And if we do that you
can see 192.168.1.0. And then the subnet
mask, of course, is a contiguous set of ones. And there are 24 ones
in this 255.255.255.0. Now every place that there
is a 1 in a subnet mask is the network address. Every place there is a 0 in the
subnet mask is a host address. That means that
the network address is masked by those ones. That's where that
subnet mask comes from. The client addresses are
not masked by the ones. They are the zeroes here. And I put the h for those
host or client addresses. And because we've got
eight separate bits that are listed for host addresses,
if we do our calculations eight bits means there
are 256 possible places that we could have in there. We always subtract two. We subtract one number
for the network address and one number for the broadcast
address, which then leaves us with the maximum
number of hosts that we can have on that subnet,
which means in this case there would be
254 separate hosts on this particular subnet. Every network is going to have
a network or subnet address. And every network is going
to have a broadcast address. And you can calculate the
subnet address and the broadcast address by setting the bits
in the host part of the mask. If you set all of
the bits to 0 you can calculate the
subnet address. And if you set all
of the bits to 1 you can calculate the
broadcast address. So let's look at this
scenario, 192.168.1.165, which is a host on a network. And this host has a subnet
mask of 255.255.255.0. And the first thing we'll
do is to calculate out what this network address might be. So we're going to have all
of our host bits set to 0. And we're going to calculate
out what that number is. And we do this by performing a
function called a bitwise and. That means we're going to
look at the differences between these bits and make
a determination of what those differences are. If the two bits are 1 and 1 that
means that we bring down a 1 into the answer that we have. If it's anything else--
if it's a 0 and a 0, if it's a 1 and a 0,
if it's a 0 and a 1-- then we put a 0
down at the bottom. The only time we would
put a 1 is if we have a 1 in both sections of these. And you can see a 1 and a
1 will bring down the 1. Here's another pair of ones. Any time that's different,
like a 0 and a 1, it's a 0. And any time there's a
0 and a 0 it's also 0. So if we list out all the
bits in our IP address and all of the bits
in our subnet mask and we perform this
bitwise and, we're left with this final number. And that is going to
be our network address or our subnet address. And if we calculate that back
to decimal, it's 192.168.1.0. That is our subnet,
or network, address. To determine what the broadcast
is for a particular network you change all of those
host values to a 1 and you perform the
same calculation. So if we have the same number
with ones in the host place, we get 192.168.1.255. And that is the
broadcast address for this particular subnet. So let's recap what
we've determined. We know that we have an IP
address of 192.168.1.165. And it has been assigned a
subnet mask of 255.255.255.0 by the network administrator. We performed our calculations. And we determined that the
subnet address was 192.168.1.0. And the broadcast
address is 192.168.1.255. The number of devices that
we can have on this network is every number that's
between the subnet address and the broadcast address. And we can calculate this easily
by looking at how many bits are available for the host. We have eight of those. That means that we
have 256 total numbers. We're going to take
out the subnet address and the broadcast address--
that's the minus 2-- which means we have a total
number of hosts that can be used on this network of 254. And if we were going
to write this out that means we would look
at 192.168.1.1-- that's the first host after
the subnet address-- all the way through 192.168.1.254. That's the address that's just
before the broadcast address, giving us a total
of 254 addresses. Let's perform the exact
same calculation again. Because it is exactly the same
calculation every single time, even if the IP
address is differently and the subnet mask
is differently. It's the same process. We're going to
convert to binary. We're going to perform
the bitwise and. We're going to change
the zeroes to ones and determine what
the subnet address is and what the
broadcast address is. Let's look at this address. 10.11.12.13 is the
device on a network. And it has been assigned a
subnet mask of 255.255.0.0. If you were to write this out
in CIDR block notation it means that we would have
10.11.12.13/16, because there are 16 bits that
are set in the subnet mask. We would know that
visually if we took these IP address
and the subnet mask and wrote it out in binary. So if we look at
10.11.12.13, this is the binary representation. And if we look at
255.255.0.0, this is the binary representation. Every place there is
a 1 in the subnet mask is the network address. And everything
that is after that are the hosts that are on
this particular subnet. So let's perform that
bitwise and function and bring down every
place there are two ones we're going to have a 1. Every place there is anything
else we're going to have a 0. And now we have
this network address and the host values that we have
associated with this network. That means if we write
this out in decimal now, 10.11.0.0 is our subnet address. And if we look at changing
all of these zeroes to ones, we would get the
broadcast address. And if we calculate
that, it's 10.11.255.255. That means the
entire range that we would have would be the first
number after the network address, all the way
through the last number, just before the
broadcast address. So let's summarize these values. If we take that number
we were looking at, 10.11.12.13 on the subnet
mask of 255.255.0.0, we've got a subnet
address of 10.11.0.0 and a broadcast address
of 10.11.255.255. That means we have
16 bits set aside for a host, which means that
the total number, if we were to calculate in binary all ones
for that 16 bits, that means gives us a total of 65,536. We subtract the network
address and the broadcast address to give us a total
number of available hosts on this network of 65,534. The first address on
the subnet is 10.11.0.1. That's the first address
after the subnet address. And the last possible host
that we can have on this subnet is 10.11.255.254, which
is the last host just before the broadcast address. As you can tell, it's a
relatively rote process to determine what the network
address is, the broadcast address might be, and the host
range of a particular subnet. But how do you know what size
to make the subnet mask for what your requirements might be? And these particular
values are often dictated by whoever's providing
you with the IP address to start with. Usually the ISP or a central
body in your organization has to find a particular
addressing scheme. They've determined what
the IP address ranges are and what the subnet
masks are that they'll use inside of the organization. And they may just hand
you a particular value and say you can subnet
this however you'd like. So you have to make a decision
about how many networks we need and how many hosts
are going to be on each one of those
individual networks. Here's one of these scenarios. Let's say in this organization
that you're at a remote site and you're in charge of
addressing your network at this remote site. The main office has
already determined what their IP addresses are. And they're simply going to hand
you a block of IP addresses. And they're going to tell you
your assignment is 192.168.1.0. We've assigned you a
subnet mask of 255.255.255.0. They might also write this
down in CIDR block notation, so it would look
like 192.168.1.0/24. Now, you know that
you have four networks at your remote location. And each network
has about 40 devices on each one of these networks. But the main office has only
given you a single network address. That means that you're
going to have to subnet it. You will have to split up this
address into smaller pieces so that you can fit
the requirements for your particular site. If we were to write out a table
of all of the subnet masks now for the IP address
that we have been provided, we can list it out this way. Here's the subnet
mask 255.255.255.0. That's the one that was assigned
by our corporate office. And we can see the binary
representation of that subnet mask, the /24 mask. They've given you one network. And you can fit 254
hosts per network. But we can start splitting
this off in different ways. If we were to subnet this
down to 255.255.255.128, which means we're just going
to use one extra bit into that last octet, this
would be a /25 subnet. That means that we could
have two separate networks. And we could have 126 hosts
on each individual subnet. Of course, in our scenario we
have more than two networks. So let's go to the next one
where we have 255.255.255.192. So now we're using
two additional bits into that last octet. If we were to write this out in
CIDR block notation this would be the /26 subnet. In this particular
scenario we've got two extra bits that we've
assigned here now for network. So we have a total
of four networks that we can have and
62 hosts per network. That particular
scheme seems to fit our particular requirements. We had four networks. And we had 40 hosts per network. Well, this would give
us the four networks. But it would give us
62 hosts per network. There's a little bit more
there than what we need. So let's look at what the
next step down might be. If we were to look at the
255.255.255.224 network, which means we're using three
more bits into that last octet, or /27, that would
give us eight networks. That would certainly give
us more than we needed. But notice that the hosts
per network would be 30. And we have 40
devices per network. So if we were going to subnet
out that single address they gave us we would
subnet it with a /26, giving us four networks
and 62 hosts per network. Let's now look at this
address in a binary form to really break out
what we just did. Here is our IP address
192.168.1.0, our subnet mask of 255.255.255.192. Here is the IP address of the
network that we were given. And here is the subnet mask. And I've broken this
out into colors. So we can see what we change
with this subnet mask. We were given the
subnet 255.255.255.0. So I took the original
subnet and made it blue. We subnetted further. And that additional subnet
we use two additional bits. And I've colored those purple. And then we have
the host bits that are now left over
after performing our particular subnet. And if we look at that we
decided on the 255.255.255.192. Here's the binary
representation. The /26 network is what that is. That will give us four networks. And it will give us
62 hosts per network. Because we have been
assigned a subnet we can't change
any of those values that we were given in
that original subnet mask. So that blue value here, the
blue bits, will never change. The only ones that we can
change are the purple ones. Those are the additional
subnets that we use. And because we subnetted two
additional bits we can turn on and off those bits to give
us four separate subnets, a 00, 01, 10, and 11. And if we were to calculate
out this particular subnet, 00, and if we had all of those
host bits as set also to 0, we would get a
subnet with a dot 0. If we had one set to 01 and all
zeros it would be a 64 network. We would have a 128 network. And if both bits were set to
1 it would be a 192 network. And if we were to look at
the number of hosts available we have these six bits
left for the host. That gives us a total number
of hosts of 64 per network. Of course, we don't use the
subnet or the broadcast, which means we have
a total of 62 hosts that we can have on
each individual subnet. If we were to
summarize everything, we could write out every
single one of these subnets now in binary and
perform the calculations. So we have this
192.168.1.0 network. We have the subnet mask
of 255.255.255.192. There's our numbers
there at the top. Now we can just
start calculating. Let's set our subnet mask in
purple here to 00 and perform the binary calculation, which
gives us the first network of 192.168.1.0, because
they're all zeroes here. And of course, if we change this
to have all ones in the host location, we can calculate
the broadcast address. And if we calculate this in
binary it's 192.168.1.63, which is 00, 11, 11, 11. Now of course, our
first host is going to be one above the
network address. So that's going
to be 192.168.1.1. And the maximum host,
the highest number host, is going to be just one
under the broadcast address, or 192.168.1.62. And I put the entire
binary down so we can reference that as well. Now if we change the 00 to
01 we can perform the same calculation for
the second subnet. If we change it to a
10, same calculation again-- and of course
a 11-- and get exactly the same calculation again. There's no other ways that
we can change those two bits. Those are the four
networks that we get. And the second network
is 192.168.1.64. We have a 192.168.1.128,
and finally a 192.168.1.192. We can now put all of
these numbers in place. And we'll assign each one
of these router interfaces one of those IP addresses. So we have 192.168.1.0,
1.64, 1.128, and 1.192. These all go to these
separate networks. And now we have four
separate networks. And we can put up to 62
different devices on each one of these individual networks. The rest of the network sees
the original subnet address. All of the routing still
continues to work normally, except now locally
on our local router we've defined what the
individual subnets might be. And by the time that
traffic gets here, this router will know exactly
where to send that traffic.
