In the last lesson,
we covered a lot of details about subnets.
Now what I want to do in this lesson
is take some of these problems and go through them
because you may experience them come test day.
So I want to walk through how to tackle them.
Now before we get to the first problem,
the first thing I want to do
is put my chart back on the screen.
Now, why am I doing this?
Well, because I already told you,
you should commit this chart to memory.
And when you get down and you sit for the exam,
the first thing they're going to do
is give you a piece of paper and a pencil.
And once they start the clock,
you can write whatever you want on that dump sheet.
And so one of the things I put down
on my dump sheet or my note sheet
is that I put down from my memory this chart,
and that way as I get sub in questions throughout the exam,
I can go back and look at those,
and so it makes it really easy.
So I'm going to give it to you here
because I'm assuming you've memorized this before the exam
and you've already written down
as you're sitting at your computer.
All right, let's take our first question:
Let's pretend you're a network administrator
for DionTraining.com.
We decide we're going to open up
a small branch office in another city.
And so to support that I need a subnet
for a private IP address that's given to you
for several different networks based on the different sizes
of the departments in this new location.
Now, the new office has been given the range
of 10.10.10.0/24,
which tells you you have 256 total IPs to work with.
Now, when you set up your new network,
I want you to configure separate subnets
for each of the departments that I'm going to give you.
And you're going to tell me which CIDR notation
is going to be the summary for each one in each place.
Now, I want you to resist the urge
of going big for each subnet.
I want you to tell me the minimum subnet
that would support my requirements.
Now, in my new company,
we're going to have this number of users that we have.
So first I have 54 new people in IT,
I have 32 new instructors,
I have five people in sales,
three in administration,
and then there's going to be an unused portion that's left over
and you're going to gimme a subnet for that as well.
Now, each of these five areas,
I need you to tell me what the CIDR notation is for it,
and that's how we're going to answer this question.
So first, we're going to take it step by step.
Again, you've memorized your table,
so this is going to be a pretty simple problem
because we don't have to do the 2 to the S
or the 2 to the h - 2 calculations
because we already have all that done in our table here.
Now, we're going to round up our department numbers
into multiples of two or factors of two.
So, it's either going to be a 1, a 2, a 4, an 8,
a 16, a 32, 64, or 128.
Now remember for each of these,
I told you how many clients need to be on the network.
For example, I say I had 54 people that work in IT,
but you need to still account for the network address
and the broadcast address for each of those networks,
that's always going to be your first and last IP.
A lot of students forget that
and they mess up the problems that way.
So if I had 54, what do I need?
I really need 56.
Now, if I move that up to the next closest range
of two to the something power, I get up to 64.
So, if I look at IT,
I'm going to end up having 64 IPs for that subnet,
and we have to figure out the slash notation for it.
If I look at the instructors,
we had 32 instructors,
and we had to get two more, which puts us at 34,
which, rounding up, would get me back up to 64,
the same as IT.
Notice that trick there,
I would expect to see something like that on the exam
where they put something where they tell you
I need 16 or 32 because that looks like you'd just be able
to give them that slash notation.
But if you forget that, first and last,
you're going to miss that up.
So don't do that, right?
Make sure you pay attention.
Now the next one we had is sales,
and for sales there was five people.
So you add two to that and that gives me seven.
And now I look at it and go, oh,
seven's close to eight,
so that's going to be eight people.
Next we're going to look at admin or administration.
Administration has three people
plus two more gets me to five.
Round that up, that gets me to eight as well.
And so that's the way we do this, right?
Now lastly, I have to calculate the unused amount.
Now, this is going to be a little bit different
because instead of rounding up,
we're going to have to actually round down.
So first we're going to start
with the total number of IPs that we had
and we were given a /24 subnet.
So that means we had 256 total IPs.
Then I'm going to take away the 64 I had for IT
and the 64 I had for instructors,
then I'm going to take away the eight I had for sales
and the eight I had for administration.
And what I get left is 112 left,
but I can't round up here
because I'm talking about what was left.
So, I have to round down to the closest power of two,
which in our case is going to be a 64 again.
So, while this problem originally looked really tough,
it really isn't
because we only have to calculate two things.
We need to figure out what's the CIDR notation for 64
because that's where we're going to use for IT,
the instructors, and unused.
And then we had sales and administration
that each had eight.
So I need to figure out the CIDR notation for eight.
And so if I figure those two out,
it's pretty easy, and I have my chart.
So if I look there for 64, that's a /26.
So if I have /26,
that's going to gimme 64 for the instructors,
64 for IT, and 64 for the unused.
Then I have sales which had eight,
and that's going to be a /29.
And administration again is eight, so that's a /29.
And so you can see this really complex problem
became really easy as we broke it down step by step.
All right, for our next question,
we're going to make it a little bit simpler.
The last one may have been something you might see
in more or less of a simulation or a PBQ.
We have to calculate lots of different things
and then pick the right ones.
But during the test itself,
it's going to be multiple choice.
So you're going to get a problem
that looks something more like this.
Let's say how many assignable IP addresses exist
in the network of 172.16.1.0/27?
Now, before we answer this, let me ask you this,
does it even matter what the IP address is?
No, it really doesn't.
The only thing that matters here
is that this is a /27.
The IP address itself is just there to mess with you.
So you can ignore it in this particular problem.
The question becomes how many assignable IPs are there
in a network that uses a /27?
Well, I'm going to have to think about that here for a second.
Oh wait, I really don't have to think about it too hard
because I have my chart, right?
Is it 30, 32, 14, or 64?
Well, looking at my chart,
I know it's either going to be 32 or it's going to be 30
because a /27 shows me there's 32 IPs.
Now, which one is it?
The keyword here is "assignable."
The question said how many assignable IPs?
Assignable IPs means that I can give it to a client,
I can assign it to a computer.
I can't give away my network ID
and I can't give away my broadcast ID.
So, out of those 32 in the subnet,
I only have 30 that are assignable.
And so my answer here is 30.
Now, even if you couldn't remember that,
using your chart, you'd still get it down
to either of those two options, 30 or 32,
and you have a 50-50 shot of guessing, right?
So even if you couldn't remember
the fact that you have the network and broadcast ID,
which I really hope you do,
'cause I've been beating that into your head
a couple of times now,
you still would at least get a 50-50 shot on this.
Now, let's assume you forgot to write down your chart,
how could you calculate this using your subnetting formulas?
Well, we would do 2 to the h - 2,
and that's going to give you your number of usable hosts.
Since we had /27 as our CIDR notation,
it becomes 32 - 27,
giving me the number of host bits, which is five.
So, two to the number of host bits,
two to the fifth is going to gimme 32,
minus two gets me back down to 30.
Now whether you want to do it from the formula,
which is a lot more work or memorizing my table,
either way will get you
to the right answer of 30 usable IPs or 30 assignable IPs.
Now just remember,
are they asking about assignable or usable IPs,
or are they asking about total IPs?
Depending on how the wording is in the question,
that's going to determine if you're going to minus two or not.
If they said, "How many total IPs exist in this network,"
the answer would've been 32.
But because they used the word "assignable,"
you have to take into account the network ID
and the broadcast ID,
and this is going to make sure
that you get to the right answer.
Now, let's go ahead and look at our third practice problem.
How many assignable IPs exist in the network?
If I have the network of 192.168.1.0/28,
this is just like the last problem.
If the /28, we're going to look at our chart
and come up with 16.
But they said assignable,
so we take off two, one for the network,
one for the broadcast, and this gives us 14.
And so our answer is 16 minus 2, which gives us 14.