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I'd like you to imagine that
a friend walks up to you

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and in casual
conversation they ask you,

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can you tell me,
says your friend,

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about a mask exactly
what it does?

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Now if you're saying, oh,
Keith, I know, I know.

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It absolutely identifies out
of an IP version 4 address,

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it identifies which portion of
that IP address is the network,

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and everything else beyond that
is the actual host addresses.

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So here, my friend,
are the details

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behind how the mask identifies
which portion of an IP address

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is the network.

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So I ran IP config on
my Windows machine,

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and this is what it gave me.

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It said my IP address
is 192.168.1.15.

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Fantastic.

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So I broke that into binary
just for display purposes.

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And the first number of 192,
I put in red in binary here.

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168, right here in blue.

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The one in green, and the 15.

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So there's the decimal
notation that we normally use,

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and the binary equivalent
of each one of those.

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Now what I'd like to do
is talk about the mask.

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There is also a 32-bit mask
associated with each IP version

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4 address.

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So on the output of
IP config up here

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it says that my mask,
and that also brings me

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to a point of nicknames.

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For example, sometimes it's
referred to as the mask.

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Sometimes it's referred
to as the network mask.

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Sometimes it's referred
to as the subnet mask.

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But any way you slice it, when
we're talking about IP version

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4, it's referring to this device
that identifies which portion,

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going from left to right, which
portion of that IP address

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is the network, and which
remaining portion is

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available for host addressing.

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So this is how it does it.

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That 32-bit mask is also
represented in dotted decimal.

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So I have the mask right here.

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There's 255.255.255.0.

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So that's the mask literally off
of the computer that I'm using.

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For display purposes, I
converted the mask octets

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into binary.

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So there's the first 255, and
the second 255, the third 255,

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and of course, this
last octet of all zeros

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represents that
decimal 0 right there.

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So here's the magic.

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The computer lines
up all the bits

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of the actual IP address with
those same bits in the mask as

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shown here.

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And so here's what
the mask does.

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We simply follow the
bits that are on.

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The bits that are
on in the mask is

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representing which portion of
the IP address is the network.

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So if we simply follow the
mask and the bits that are on,

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and then stop when they
stop, which in this case

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happens to be right
here, that my friends,

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is the dividing line.

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Everything to the
left is the network,

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and everything to the right
is the actual host portion.

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So in this case, we could
say that my computer is

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on network 192.168.1,
the first three octets,

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and that my host address
is .15 on that network.

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The way it actually
determines this

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is by doing some multiplication.

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It multiplies the
bit in the mask

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against the corresponding
bit in the IP address,

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and if we did that, for example.

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1 times 1 equals 1.

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and 1 times 1 equals
1, 1 times 0 equals 0.

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Effectively, we're going to have
the same exact number that we

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have up here down below.

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And the very last
octet, what it's

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going to be if you
multiply all those by 0,

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they're all going to
be zero no matter what.

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So if we converted
this back into decimal,

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it would be
192.168.1.0, and that's

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how we represent networks
inside of IP version 4.

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And more specifically, we
can put a slash here, and say

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24, which literally means that
the first 24 bits, 8 bits here,

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and 8 bits here,
and 8 bits here,

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those first consecutive 24
bits, going from left to right,

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represent the network,
and the last 8 bits

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are available for
host addresses,

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or like house addresses
on those streets.

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I've got a little exercise
I'd like you to do with me,

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and that is this.

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Let's pretend that this mask
isn't this one right here,

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so we're going to replace
that for a moment.

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And let's say that the mask
instead is 255.255.0.0.

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So the mask in this octet
would all be 0, just like that.

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And here's my question for you.

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Now that we have this mask, what
is the actual network address

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if this is our mask, and how
many bits of the IP address

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are being used for network?

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And how many bits are
now available for host

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addressing on that network?

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So take a moment right
now, go ahead and pause me.

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Determine what you
think the correct answer

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is for the actual
network address,

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and then we'll compare notes.

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Awesome.

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Thank you for taking
a moment to do that.

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So here's how I
would look at this.

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I'd say, well, we're going to
follow the bits in the mask

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until they stop, and then you
can draw a line, and everything

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to the left that is the network,
and everything to the right

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are the host addresses.

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So we could say that with this
mask, the network is 192.168,

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and then, anything beyond the
mask when we're identifying

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the network, we just 0 out.

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So this is the 192.168.0.0
network slash 16.

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It's a 16-bit network, and we
have another 16 bits available

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for host addresses
on that network.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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