All language subtitles for 25 - Routing - Variable Length Subnet Masking (VLSM)-eng

>> Well it's time to culminate
the subnetting topic.

As we look at the final method,
variable length subnet masking.

And I know some of you might be looking,
going, "Well, there was four examples

with the other one, what's
up with the one example?"

Well trust me, when you see it, you're going
to be like, "Okay, I only want to do one

of those," 'cause it takes some time.

And really, all VLSM is, is using the skills
that you've learned and all of a sudden,

they think we've done up till now and
just kind squeezing them altogether.

There's not-- I won't say there's not really
any new subnetting topic that we talk about,

just kind of applications
of what we've already seen.

And here's the concept.

Let me explain the concept and
then we'll get into the example.

The concept is simple.

It's changing your subnet mask
whenever and wherever you want.

So for example, we've been doing so far in all
of our subnetting what I would consider kind

of like classful subnetting to where we
find one subnet mask to rule them all.

We say, "Okay, I want 50
networks" and we break it up

and there's our one subnet
mask that rules them all.

Or "I want 20 host per network," there's
our one subnet mask, rules them all.

We just find one subnet mask.

But I'm looking at the example,
as you've probably seen situations

where will this office has 50 users and this
office has 10 users and our WAN link has two,

I mean there's two devices right there.

So finding one subnet mask to rule them
all might not be the best way to go

because we'd have to figure it out
base on the largest subnet, right?

But then-- and for finding it base on that,
then using that subnet mask here use--

waste a whole bunch of IP
addresses and same thing here.

So VLSM is really just saying well, I want
to find one subnet mask for that network,

one subnet mask for that network
and then pushing it all together.

So now let's move in to our example of VLSM.

Take this network environment.

We've got three networks, oh,
we'll say three user networks

of 20 users, 20 users and 60 users.

And then we've got three networks
of WAN links that we can't forget

about that would have like
two devices each, right?

So here's our scenario and this is how you'll be
able to tell if we're getting a VLSM question.

It says subnet 192.168.1.0/24, so class
C, to address this work, here's the key,

use the most efficient addressing possible.

That immediately tells you, "Okay, we
want those subnet masks to be efficient.

We don't want to waste IP addresses if we can."

So the key behind subnetting or sub-- VLSM is
to do the same things that we've been doing.

But if we want to be efficient,
start with the largest subnet first.

Okay, largest subnet, what is it on the screen?

Identify this guy right here, 60 users.

So it's using the same methods that we've seen.

So step 1 and I'll go a little faster 'cause
we've seen them in the previous nuggets.

Step 1 is convert that number to binary or
more specifically, find out how many bits

that number needs to be represented in binary.

So I'll throw our binary chart down
here, 128, 64, 32, 16, 8, 4, 2, and 1,

so 60 in binary is going
to be, let's see, 0, 0, 1.

So meaning I can stop there because I
know that 60 is going to take 6 bits.

Remember, I can't get the number
60 with any less than 6 bits.

So step 2, we say, "Okay, reserve bits
in the mask and find our increment."

So I'm going to say our mask originally was a
/24, right, which is 1, 1, 1, 1, 1 or dot, dot,

dot, dot, dot, a whole bunch of ones, 24 ones
dot and then 1, 2, 3, 4, 5, 6, 7, 8, the zeros,

I want to write out 'cause that's my playground.

This represents the first three
octets of 24 bits that are there.

Okay. So reserve bits in the
mask and find their increment.

Now-- so I'll ask the question, are we
creating more networks or is there focus

on the networks or are we saving the host?

Saving the host, absolutely, because we're
focused on 60 users, not 60 networks.

I need 60 users, 60 IP addresses on this network
so I'm going to go right to left, 1, 2, 3, 4, 5,

6, and the rest can go, whoa, 7, 8.

Did I-- 1, 2, 3, 4, 5, 6, 7, no, I must
have just wrote skinnier or something.

So we've got-- this is our new
subnet mask for the network.

So if we were writing bit notation be /26

or decimal 255.255.255.192 would be what
this is converted back to a decimal number.

So that's what our subnet
mask is for that network.

That's my emphasis, our subnet
mask for that network.

So now we find our network
ranges or wait, our increment.

Lowest network bit converted back
to a decimal number, 64, right?

And that makes me feel good
because I was after 60 users

and I got the number 64 as my increment.

So step 3, we start off with 192.168.1.0 and
then we just start adding 64, 192.168.1.64.

Now stop the train right there.

Let me ask a question.

How many networks of 60 do we need?

Look at it.

Just one, right?

That's it.

That's the only network of 60 that
I have, so I can stop right there

because I got my one network of 60.

It's this one, 192.168.1.0 through 63 'cause
the next network starts at 64, so I'm done.

Now, what I've done is created a
unique subnet mask for that network.

Let's right it up there.

So I'm going to say, let's
go blue, ah, let's go red.

I didn't get the right pen.

192.168.1.0 through 63/26.

Now, I write it this way because I like to
see it, but know that that's not a standard.

You don't see that, you know,
that they'd be like, "Hey,

that's not a standard way of writing it."

I like doing that because I can see the range
and the subnet mask all at the same time.

So that tells me what that network is.

Okay. Great.

So take this, put into a
little box, and do it again.

What's the next biggest network that we have?

20 users, right?

So let's just jump up top.

I like keeping them on there 'cause
I want to show you how this works.

So 20 users, if I do that, how
many bits to get the number 20?

No, no, no, yes.

Okay, so line moves over.

It takes 5 bits to get the number 20, okay?

Step 2, reserve bits in the
mask and find my increments.

Same subnet mask.

Nothing different here, it's as if
we had never done subnetting yet.

So /24 is a whole bunch of 24 ones
dot 1, 2, 3, 4, 5, 6, 7, 8 zeros.

So now, I'm going to say, "Okay, am I
creating more networks or saving the host?"

And the answer is we all
scream, saving the host.

We need to save 5 bits of host bits,
otherwise, I can't get the number 20.

1, 2, 3, 4, 5, 1, 2, 3 leftover, that's our
new subnet mask, /27 or 255.255.255.224.

So those are two different ways
of writing the subnet mask.

Our increments, let's grab green
here, our increment is going to be 32.

Once again, makes me feel good because
I'm like okay, 32 is bigger than 20.

So step 3, okay, here is where it gets
a little weird, find our network ranges.

Okay, so 192.168.1.0, .1.32, .1.

Okay, wait a sec.

We're getting our increments
and I look, I need two networks.

One and two of 20 users but
do you see any problem here?

The first two overlap with this.

If I tail on the end ranges,
this goes through 63.

The first two overlap with this.

This is the only thing different with VLSM.

I look at those first two and I get--

I go, "I'm sorry, I can't use
those because they're overlapping.

They're already uses so I guess a better rule to
say, a better way of thinking about it is rather

than just, you know, writing
these and doing that,

you just pick up right where
the last one left off."

So I didn't have to write this.

Now I did because I want you to see it

but I could have just started
from 64 and felt safe doing it.

That's the beauty of the powers of two.

If I count by 128s, 64 is going
to hit every value, right?

If I say, 0128, 250-- I guess 256 would
be the number, then if I count by 64,

I'm going to have 064, 128, I'm going
to get there, and then 192 and then 256.

I'm always going to hit the same value.

And then if I count by 32s, I'm
going to get 032, 64, 96, 128,

you're always going to align these things up.

You're always going to--
they're all powers of each other.

They're all powers of two,
so you're always going to--

I mean, so that-- I'm showing you that.

You're like, "What's the point?"

I'm showing you that so that when you do this
and you find your one range and you say, "Okay,

that's where the next one start."

As long as you begin with the biggest subnet,
you can always feel safe starting there,

you know, just saying you know what,
I don't even have to right that.

I'm just going to go 192.168.1.64
and then I start adding 32 to that,

so 1.-- well that'll be 96, 1.128.

And I would say, "Okay, well I can
stop there because I got my two ranges,

64 through 97 is my first one,
96 through 127 is my second one."

I've seen how this works so now, I've
done my second subnetting problem.

Let me just clear off a little space
in here so I can look nice and clean.

So I've done my second subnetting problem
and I can come here and say, "Okay,

well this one, let's go back to red."

This one is 192.168.1.64 through 97/27.

That's this network.

And this network over is
192.168.1.9-- what do we have?

90-- wait, what am I doing?

What trickery, witchery is this?

And-- I can't count, 97 is-- that's not good.

[laughs] I heard some of you are
like, "What's Jeremy doing now?"

No, this goes through 95, right?

95. My goodness, sorry.

Hang on. Hang on.

Little dissection here of some bad numbers.

So 97, so this is actually 95, right?

So this one goes from 96 through
1-- I'm making sure, 127/27.

Does that feel better?

I know-- okay, I'm sure I disturbed
a number of people by doing that.

Okay, so that's this network.

Now, before we do the last piece 'cause
there's one more, it's the WAN links, right?

First thing I want to say-- I want
to ask you, are you getting there?

Are you kind of seeing how this works?

And you'd see why it only takes one example.

You're kind of like, "Okay, I get it.

It's multiple subnetting problems all in one."

That's what VLSM.

But I want to ask the second
question that I get ask quite a bit.

A lot of people that are
certification minded are like,

"How do even ask a question
like this on the exam?"

Like I mean, obviously, the exam is a serious
of multiple choice, drag and drop, you know,

simulation, all that kind of stuff,
how do they ask a question like this

if they were going to ask me to do VLSM?

What will typically be used is drag and drop

to where what you'll see is a diagram
just like I'm showing you right here.

And then you'll see a whole bunch of boxes.

And they'll just have subnets, subnet,
subnet, subnet, subnets, all of these subnets.

So one box will be like 192.168.1.64/27.

One will be, you know, .96/27.

And you actually have to drag and drop those
boxes to the specific areas of the network

where they belong and you
just kind of plant them there

and that's how it knows whether
you got the right answer or not.

So, now, and also keep in
mind, the test is geared.

It's smart enough to know, you know, for
instance, these two are interchangeable.

I could have put 96 through 127 over
here and I could have put 64 through 95

over here, and that's totally fine.

That's not a problem, so good.

Are you ready to do the last piece?

Let's see where we can squeeze
it in 'cause I want--

I like keeping them all on there
so you can see what we've done.

Let's go small.

So I'm going to go down here one.

So the last thing is we need a
few networks for our WAN links

of two host each, so let's start there.

Two host, what's the binary equivalent of 2?

Right there, 2 bits.

To get the number 2, I need 2 bits.

So step 2, original subnet mask
/24 equals a bunch of ones,

and 24 ones, 1, 2, 3, 4, 5, 6, 7, 8 zeros.

So I say, "Okay, I need to reserve my bits
in the mask and I'm going to save the host."

I need two host per network, 1, 2 are saved.

1, 2, 3, 4, 5, 6 flip over to
the network side, the dark side,

I now have a /30 as my subnet
mask or-- I'm not going to write.

Or let's write over here, 255.255.255.252
is the subnet mask so that is.

So again, two ways of writing the same thing.

So I'm going to come over here and I'm going
to say my lowest network bit convert back

to a decimal, that'd be 1, 2, 4, that's a 4.

So now, my network range is
we'll go up here, how is that?

So I'm going to start off
with 192.168.1.0, right?

And I just start counting by 4s.

8, 12, now stop the train, that's
going to take a while, right?

'Cause I look at all of these, 0 through
3, 4 through 7, 8 through 11, and I'll--

as I go down, that's going to take some
time because these guys are all used up.

They're overlapping with
this range or these ranges.

I mean, I would have to count by 4s down to 128

or what if we just went dot,
dot, dot, 192.168.1.128?

Like I said, if you count by 4, you're
going to hit every increment of 16, 32,

64, you're going to hit all those.

So it's very safe to say we left
off at 128, let's start there.

128, 132, 136, 140, now,
how many networks do I need?

Three. Good, I got them.

128 though 131, through 135, through 139.

So now, I can go line these guys up.

So top link, top WAN link is going
to be 192.168.1.128 through 131/30.

Over here, 192.168-- hey, let
me just write them all up.

Okay, there.

I've written them all up.

So I've now created networks
for each one of those WAN links.

Let me just put there-- all there.

Now, then you might see, you know, on the exams
something that has you drag the networks just

like I've done and kind of, you
know, this would be your empty boxes

and you would drag the networks
in there and label them.

Other ones might actually have you assign them.

So I mean, they might say, okay, this side
would .129, this side would be .130, you know,

so those are the two useable IP addresses
from that network or over here, you know,

our default gate way might be 192.168.1.97.

You know, the first valid IP address from
here and that's what everybody points

to as their default gate
way, maybe the computer.

I mean, a computer can be anything but I'll
do that 1.-- I'm out of room, 98, you know,

/27, you know as the subnet mask.

So it depends on how that works, you
know, what the question is looking for.

Many different ways of asking it
but do you see how we've done that.

Isn't that amazing though if you look at it?

We've address this whole
network and barely gone over half

of the total IP addresses that we have.

I mean that's the beauty of VLSM.

[laughs] Now, let me ask this question.

What if, you know, I said
this is 60 users but look,

our subnet mask only allowed
for, what is that total, 62?

What happens if this office grows
and they hire five more people?

Your host because you explode
your whole scheme, right?

You just outgrew the subnets and
now you're like, well, you know--

yeah, that's where it just gets ugly.

So while VLSM looks awesome
on paper and I don't--

hang on, don't throw the
baby out with the bath water.

VLSM is awesome and it's used all over
the place but it's not used in such a way

to put your subnets so tight
that there's no room for growth.

You want to allow room for growth but
you just don't want to be at a place

to where you're forced to waste IP addresses
because of these giant subnets you had

to create for one area of your network.

So do you see why I said we'd
only need one example of it?

VSLM really isn't anything new.

It's just doing what we've already done
again and again and again and again.

I think, you know what, the newest thing is just
to say, "Well, start with the biggest subnet,

you know, and kind of work your way down so that
you get efficient use of your IP addresses."

But that is it, that's the
culmination of everything subnetting.

The cool thing is at this point, you-- there's
nothing that somebody could throw at you

from IPV4, you know, from normal TCPIP, where
they're like, "Hey, check out this subnetting."

I mean, you've seen it all.

You've seen the binary behind it.

You've seen how to do it on networks, on
host, reverse engineering and then finally,

getting super efficient using
variable length subnet masking.

The only thing that I would add to
this is this is truly classless,

not like you're a lesser person.

But this is truly classless IP networking.

There are, as we dive into routing
and get into routing protocols,

there are some routing protocols, old routing
protocols, like RIP version 1 and things

like that, that don't do this kind of thing.

They don't have the ability to do
variable length subnet masking.

They can do subnetting but once you find that
one mask, they have to use that for everything.

But that being said, nobody
uses those protocols anymore.

So this is now a normal way of life.

I hope this has been informative for you
and I'd like to thank you for viewing.