You may see a subnet mask written
in a number of different ways.
And in this video, I want to show you
how to convert between all of these different
representations.
You can think of the subnet mask,
though, as a contiguous series of ones, where the ones are
on the left side of the subnet mask,
and the zeros are on the right side of the subnet mask.
So if we have a subnet mask, like this one that
has three octets of all ones, we know that this subnet mask
is 255.255.255.0.
You might also see this subnet mask written as a /24.
That means there are 24 bits in this particular subnet mask,
in these first three octets.
This is also called a CIDR block notation.
C-I-D-R that stands for Classless Inner Domain Routing.
You might also hear this referred to as a prefix
notation or slash notation because we use the /24
to specify the number of bits.
Because we use this slash to designate
the number of bits that will be used for the subnet mask.
So for a subnet mask of /24, we know that that is the size
of the network and the remaining bits,
which would be eight bits, in this case,
would be the number of bits available for hosts.
Let's perform some conversions between
the binary representation of the subnet mask and the CIDR block
or the slash type notation of the subnet mask.
I have a subnet mask here written out
in binary, which has all ones in the first two octets
and all zeros in the last two octets.
What we're going to do is add up the number of ones in this
entire subnet mask, which means that we would have 8 plus 8
plus 0 plus 0, which means there is /16--
16 bits that happen to be in this subnet mask.
That means that our network is going to be 16 bits long,
and the host values are also going to be 16 bits long.
Let's perform the same conversion
with this subnet mask, which is all ones in the first three
octets, two ones in the last octet, and of course,
zeros after that.
If we then add all of those up, 8 plus 8 plus 8 plus the two
bits in the last octet, we have a total of 26 for a /26 subnet
mask.
This means that the network is going to be 26 bits,
and the host value will use the six bits at the end.
And here's another binary subnet mask,
which is all ones in the first octets, four ones and four
zeros in the second octet, and then all zeros after that.
We then add up all of the ones, which is 8 plus 4 plus 0 plus 0
for a /12 subnet mask.
That gives us 12 bits to use for network addresses and 20 bits
to use for hosts on each one of those networks.
If you only have eight bits available in each one
of these octets, there's only a certain number
of combinations of these binary to decimal representations
you can have.
For example, all zeros would obviously be zero.
A one and all zeros is going to be 128.
Two ones and all zeros is 192 and so on.
So instead of performing a manual calculation
every time you wanted to convert between binary and decimal,
you could simply write out this chart
and very easily be able to convert between them.
So let's take the subnet mask that we used before,
which was all ones in the first octet
and four ones in the second octet and zeros after that.
We know that all ones, if we look at our chart, is 255,
and then four ones and four zeros, if we look at our chart,
is 240.
So the subnet mask would be 255.240.0.0.
We know that there are 12 ones, so the subnet mask will be
a /12 CIDR block address, giving us 12 bits for the network
and 20 bits for all of the hosts that would be on each network.
Let's use our chart to perform the same calculation
with this subnet mask.
In binary, it's all ones in the first octet,
all ones in the second octet, and three ones
in the third octet.
Well, we know that all ones is going to be 255,
and three ones and the rest 0 would be 224,
making the subnet mask 255.255.24.0.
And if we add up the number of ones,
there are /19 for this subnet mask,
giving us 19 bits for the network and 13 bits to use
for hosts on each of those networks.
Let's do this in reverse, where we have the CIDR block
notation, and we want to convert back the decimal representation
of that.
We have a /26 as our subnet mask.
If we write this out manually, in decimal form,
there are 26 ones and then the remaining zeros
for the subnet mask.
If we look at our reference chart,
we know that all ones is 255, and then
two ones at the beginning would be 192, making this subnet mask
255.255.255.192.
26 bits for the network address and six bits
for the host on each network.
And let's do one last conversion--
see if we've really got it.
We'll do a /20 subnet mask.
We'll write this out manually in binary form with 20 ones,
and the rest would be zero.
We know that the all ones is 255,
and a four ones and four zeros, looking at our chart, is a 240.
So our subnet mask is 255.255.240.0,
making the network 20 bits long and the host 12 bits long.