In the early days
of the IP protocol, there were only
three subnet masks that could possibly be assigned
to a particular workstation. There was a Class A
subnet mask, a Class B, and a Class C. The subnet
mask for a Class A network is 255.0.0.0. This also means there are 8
bits available for the network address, and 24 bits available
for hosts on those networks. A Class B subnet
mask is 255.255.0.0, which means there are 16 bits
available for the network and 16 bits available
for the hosts. And a Class C subnet
mask is 255.255.255.0, where there are 24 bits that
are available for networks and 8 bits available for hosts. If we were to then draw lines
through the subnet mask, you can see where the separation
is between the network address and the host, and
this line tends to change, whether it's a Class A, a Class
B, or a Class C subnet mask. As IP became more
popular, we realized that limiting our subnet
masks to a Class A, B, or C was not a very efficient way
to perform IP addressing. So we really haven't used this
method of automatic subnet masking since 1993. But we still use these terms
in casual conversation. We might have a network
that's a 192.168.1.1, and somebody might ask,
what's the subnet mask on this network? And we'll simply
say, it's a Class C, which is a much
faster way than saying, oh, it's 255.255.255.0. The reason we still reference
these class-based subnet masks is that they become a
starting point if we ever want to provide any additional
subnetting to a particular IP address range. Here's a summary of
the subnet classes. You only need to know Class
A, Class B, and Class C. Class A subnet
masks are associated with IP addresses that begin
with the number 1 through 126, and the subnet mask for
Class A is 255.0.0.0. If an IP address happens to
start with a 128 through 191 as the first octet, then
that is a Class B address, and the default subnet mask for
that address is 255.255.0.0. The first octet of a Class C
address will start with a 192 through 223, and the default
subnet mask for this Class C address is 255.255.255.0. As I mentioned earlier, you need
to know these default classes so you know where to start
when you begin IP subnetting. Let's look at a number
of different IP addresses and determine what
class this particular IP address belongs to. Our first one is 17.22.90.7. If we refer back to our
chart of IP address classes, we know that if the
first octet happens to be a value between 0 and 126,
then it is a Class A address. The next address
is 220.10.77.40. And again, we want to look at
that first octet, which is 220, and we know that our Class C
range is between 192 and 223. And 220 certainly fits
inside of that range, so this is a Class C address. The next address is 165.245.0.1. Again, we want to look
at the first octet, which is 165, and referring
back to our chart, we know that anything
between 128 through 191 is a Class B address. The address of 128.90.10.2
starts with a 128. We know that Class
B addresses begin with 128 through 191, which
makes this a Class B address. This next address,
191.77.24.250 starts with a 191. And again, we know our Class B
range is between 128 and 191, so this is also a
Class B address. And the last IP
address, 192.1.12.5, we know that 192 falls into
the Class C subnet range, so this would be a
Class C IP address. Let's start using these IP
addresses and subnet masks to start architecting the values
that we could use for devices on a particular IP subnet. The first address that we'll
calculate is a network address. This would be the
first IP address that happens to belong
to a particular subnet. You can also get
this value by taking all of the host bits
of a particular subnet and changing them
all to zero, and that will allow you to calculate
the network address. If the very first IP address
on a particular IP subnet is the network
address, then one that is just one number above that is
the first usable host address. So if you've calculated
the network address, it's very easy to find the
first usable host address. You just add one
to the IP address. Conversely, you can
find the network address by determining the first
usable host address and subtracting one from that. The last IP address on
a particular IP subnet is the network
broadcast address. If you were to set
all host bits to 1, this would allow you to
calculate the network broadcast address. If you then want to define the
last usable IP address that you can assign to a workstation
or some other device on that subnet, you would find
the network broadcast address and the last usable host address
would be one number lower than that broadcast address. Let's now take what we
know about the network address, the first usable IP
address, the broadcast address, and the last usable IP address
and do some calculations using actual IP values. So let's take the IP
address of 10.74.222.11, and when we perform
this calculation, let's assume that
we're using the default values or the
class-based subnet masks. We know that anything that
starts with an IP address of 10 into the Class A subnet range,
which means the subnet mask will be 255.0.0.0. Now that we know
the subnet mask, we're able to draw that
line that will separate out the network part
of the IP address versus the host part of
the IP address, and let's put those into a
particular chart. We know our network
value is everything to the left of the line,
so that would be 10, and then our host
values would be everything to the right of the
line, which would be 74.222.11. To find the network
address, then, we set all of the host bits to 0. So instead of 74.222.11,
we have 0.0.0, which means the network
address is 10.0.0.0 for this particular IP subnet. To find the first available
host on the subnet, we add 1 to the
network address, which means the first
available host could use an IP address of 10.0.0.1. To find the broadcast
address for this network, we would change all
of these host bits to be 1 in binary, which means
that our broadcast address would be 10.255.255.255. And to find the
last available host that you could use
on this IP subnet, we subtract 1 from
the broadcast address, making the last
available IP address on the subnet 10.255.255.254. Let's perform the
same calculations with another IP
address, 172.16.88.200. We first need to determine
what the default class would be for this IP
address, and if we look at the first
octet, which is 172, that falls into the
range of a Class B IP address. A Class B IP address will
then use the default subnet mask of 255.255.0.0. For a Class B address, then,
we're drawing the line right in the middle between
the 255.255 and the 0.0, which means that we have a
172.16 on one side of the line and 88.200 on the other side. With this Class B address, then,
our subnet mask separates this IP address into two sides,
where the 172.16 is our network and the 88.200 is on the host
side of this particular IP address. To find the network
address, we need to set the host values to
0, so the network address is going to be 172.16.0.0. To find the first available
host on this network, we add 1 to the
network address, which means the first IP available
on this subnet is 172.16.0.1. To find the broadcast
address for this subnet, we're going to set all
those hosts bits to 1, meaning they will be 255.255,
making the broadcast address 172.16.255.255. And the last
available IP address will be 1 fewer than
the broadcast address, making it 172.16.255.254. Let's perform one
more calculation with an IP address
of 192.168.4.77. If we look at that
first octet of 192, we know this is
a Class C address so it will have a default
subnet mask of 255.255.255.0. That means we're
going to draw the line right after that third
octet, making the 192.168.4 the network part of the
address, and the 77 as the host part of the address. To find the network
address, then, we're going to set all
the host bits to 0, and there's only a
single octet of those. So the network address
is 192.168.4.0. The first available IP
address on the subnet will be 1 value larger
than the network address, or 192.168.4.1. To calculate the
broadcast address, we need to change all
the host bits to 1, making that decimal value
255, which means the broadcast address is 192.168.4.255. That also means if we
subtract 1 from that, we have the last
available host address on this network,
or 192.168.4.254. As you can see,
if the subnet mask happens to fall in that
Class A, B, or C range, it becomes relatively easy to
calculate all of these values. In future videos,
we'll show you how to calculate exactly
the same information, even if that delineation
is not on that Class A, B, or C range
of subnet masks.
