- [Voiceover] Now that we understand how to subnet
using block sizes, it takes practice to get this down,
so let's take a look at what we have here.
Looking at this graphic you can see that
I have two host addresses, both using a /25,
which is a 255.255.255.128 mask.
These are Class C.
192, 168, 10, are my networks.
So now my host ID's are 68 and 168.
I'm subnetting in the fourth octet,
a 128 is a block size of 128 so my subnets are zero,
and 128 in the fourth octet.
The number right before 128 is 127
so I'll put that below the zero.
My valid hosts are one to 126.
I can see that this host is in the zero subnet.
128 plus 128 is 256, so the number right before that
is my broadcast address so the last subnet or 255.
My valid host ranges are 129 to 254.
And that puts this host directly into the 128 subnet.
Let's take a look at another example in this graphic.
Same IP addresses actually, 192.168.10.68 and 168,
but I'm using a different mask, /26,
which is a 255.255.255.192.
All right? So, again, I'm going to just look
in the fourth octet, which is interesting to me,
so I call it my interesting octet where my block size is.
And I count it starting at zero,
counting my block size of 64.
Zero, 64, 128, 192, 256.
All right?
So, in a zero subnet the next subnet is 64
so I'll put a 63 down here.
In the 64 subnet, the next number is 128 so I'll put a 127.
The 128 subnet, the next one is 192 so I put a 191,
and then of course we end up at 255.
My valid hosts then are numbers in between, 1 to 62,
65 to 126, 129 to 190, and 193 to 254.
We can see that the 68 is in the 64 subnet.
The 168 is gonna be way over here into the 128 subnet.
And my broadcast addresses are 127 and 191.
Let's jump over and do a /28 as our last example here.
A /28 is a 255.255.255.240.
A 240 is a block size of 16 in the fourth octet.
Cisco loves their sixteenths because
they know you count by 16.
So let's do that together.
Zero, 16, 32, 48, 64, 80, 96, 112, 128, keep going!
144, 160, 176, 192, 208, 224, 240, and then 256.
Now I'm looking for some specific hosts here,
38, 138, 238.
So I'm gonna say that the 32 network looks good to me,
right here, and then the next subnet after the 32 subnet
is 48 so I'm gonna put a 47 down here.
33 to 46 is my valid hosts, so the 38 is in the 32 subnet
with a broadcast of 47.
The 138 host here, I'm gonna come all the way over
to the 128 subnet.
The next subnet is 144, so I'm gonna put a 143 here,
do 129 to 142 as my valid hosts, so this absolutely works.
The 138 host is in the 128 subnet with a broadcast of 143.
Now lastly, the 238, we're gonna go almost all the way
to the end and use the 224 subnet, aren't we?
The next subnet is 240, so my broadcast address is 239,
valid host 225 to 238.
Now what I've shown you here is how to take a host address
with a mask and determine the network, broadcast,
and valid host range, which is your biggest CCNET objective.