- [Voiceover] Now that we understand what VLSM is,
or Variable Length Subnet Mask,
we understand that VLSM can use different masks
on different segments in the network by assigning
different length masks on our router interface.
This is referred to as classless or classless routing
and it's based on the number of hosts on that segment.
For example, looking at this graphic here,
we can see that on my point to point links here
between routers that I have a slash 30.
It doesn't matter that it's a Class A here,
we need to look at the mask.
A slash 30 is a 255.255.255.252.
This give me a block size of four
in the fourth octet
with only two valid hosts.
Right?
So 10.1.1.8, subnet eight.
My valid hosts are .9 and .10.
And this one here with a slash 30,
block size of four,
subnet is 12.
My valid hosts .13 and .14 with my broadcast address .15.
On this one here we have a slash 28.
Again, it doesn't matter that it's Class A,
it's where am I subnetting in the fourth octet.
This is a 255.255.255.240 which is our
(chuckles) entire block size 16, isn't it?
128 subnet.
That gives me 129 and the next increment of 16 is 144,
so my broadcast is 143.
So I'll put a 129 here on the routers.
My first interface.
Doesn't have to be that host address,
but in this case here I'm going to use
the first one in the range.
And I can go from 129,
or at this point here now that I've used 129,
130 to 142, 143 is my broadcast.
And as the last example in this graphic,
we have a slash 27 which is block size of 32 starting at 64.
So the next subnet is 96.
My broadcast is 95.
So, I'm 65 to 94.
So if I put a 65 here,
I could still have 66 to 94 that I could use
as valid hosts here.
So notice how I am just assigning just the right
amount of hosts needed.
Now to make sure that you don't overlap,
let's take a look at one more thing.
Overlap meaning it's really easy to put one block
on top of another if you're not paying attention
and you'll get duplicate addresses.
In this example here,
we can see that I have some different sized networks
that I need in this example here.
I need a block size of 64.
Now these hosts are with twice the amount needed,
so that gives us a lot of growth.
30 hosts here.
So this one here's a block size 64,
do this one with a block size of 32.
This one's going to be a block size of 16.
This one be a block size of eight.
And it looks like I have three block sizes of four
coming up as well.
Now I'm going to start at the very beginning, at zero,
and say, "Hey, 192.168,"
again that's what we're using here,
"10.0 is my subnet,
"but what's my mask I need to use here to give me
"a block size of 64?"
That would be a slash 26 or 192 mask in the fourth octet.
My valid host would then be one to 62,
zero is my subnet.
64 is the next subnet.
Broadcast 63, valid hosts one through 62,
all using a slash 26 mask.
The next one is a block size of 32,
so now this ends here.
So the reason I put it in this chart,
so I know where it ends,
and it's going to end at 63.
Zero's my subnet,
63 is my broadcast.
So I'm going to start at 64,
I need a block size of 32.
So the next block of 32 is 96,
so I'll put a 95 here.
That's my broadcast in the last number here.
So my valid hosts are the numbers in between
the network address.
So I'm going to start at 10.64,
that's my network address,
broadcast 95.
So I have my valid hosts here are 65 to 94,
all using a slash what?
Slash 27 gives me a block size of 32.
And then a couple more here,
block size of 16 needed with 14 hosts.
I can start at 96.
96 and 16 is 112,
so I end at 111.
192.168.10.96 is my subnet,
and this is a what?
Slash 28.
So this is a 96 subnet with a slash 28.
My valid hosts are 97 to 110,
my broadcast address is 111,
and all hosts use a slash 28 on this LAN.
I have one more network that's a block size of 8,
and then I have three block sizes of four,
so I'm going to start at 112 with a block of 8,
ends at 119 and the next block is 120.
My broadcast is 119.
192.168.10.112 and a block size of 8
is going to be a slash 29.
It's my network address.
Okay, the 192.168.10.112 is my subnet address.
With a slash 29,
gives me a block size of 8.
So my valid hosts of the numbers in between
the network and broadcast, 113 through 118,
all of them using a slash 29.
Now I have three block sizes of four.
I could easily start here at 120.
I could also start from the bottom and move up too
if that's what you prefer.
120 ends at 123,
so my valid hosts will just use this first one here.
There's 121 and 122.
All of them using a slash 30 here right, aren't they?
The network address is 192.168.10.120/30.
And my next one is 124,
192.168.10.124/30,
and that gives me valid hosts here of 125.
These are pretty easy to figure out, aren't they?
And 126, broadcast 127 against slash 30.
The last one's going to be the 192.168.10.128/30.
So that gives me 129 and 130,
with 131 being my broadcast,
and that's where I end.
And I still have now all this open space,
from 132 to 255,
that I can assign to the hosts.
Now the difference here, before we end this,
I really want to make sure that you understand
that if I tried to this in a classful manner
using one mask, say a slash 26,
I wouldn't have enough subnets.
If I then went to a slash 28,
which would give me enough subnets,
I wouldn't have enough hosts for everything.
So by doing classless in VLSM,
I'm able to efficiently subnet this network.