This is one of multiple IP addressing and subnetting scenarios rather than just working out the subnets
for a scenario.
We're gonna work them out and then configure the devices so that we can practically design and configure
a Cisco IP network. In this scenario
let's assume that we've got two sites one in San Francisco one in New York.
The sites are connected via a serial link.
In this scenario, we've been allocated a subnet of 192 168 1.0 slash 24.
And we need to subnet this network to support this topology.
Let's assume that this is a small medium business.
So, all PCs at each site will be in the same subnet.
We're going to keep it simple by configuring the hosts in a single subnet at each site but there are
60 hosts in San Francisco and 60 hosts in New York.
OK, so this is our subnet 192 168 1.0 slash24, now slash 24 means that there are 24 binary ones
in the subnet mask. In the first octet
we have eight binary ones.
Second octet to 8 binary ones a third octet a binary ones.
Now in an octet there is no space between the binary ones.
I'm just going to represent them that way to make it easier to read 24 is a side notation or classless
into domain writing notation.
Twenty four means that the subnet mask again consists of 24 binary ones.
Again there is no space between the binary ones.
But to make it easier to read.
I'm going to show it with spaces.
So he has octet 1 8 binary ones equates to 255 and decimal 8 binary ones equates to 255 8 binary ones
equates to 255 8 to binary zeros equates to 0 using decimal.
So this is the dotted decimal notation of the subnet mask.
This is the Sajda notational binary notation.
Now when working with binary Don't forget the following.
If you have a 1 in the least significant but in the octet that represents 1 in decimal 1 in the second
least significant position in binary equates to 2 in decimal that equates to 4 and so forth and so 1
1 in the most significant position in the octet equates to 128.
So if we look at 1 9 2 1 6 8 1 2 0 in binary 1 9 2 is that.
In other words it's 128 plus 64 168 in decimal.
Looks like this in binary.
In other words it's 128 plus 32 plus 8 1 in decimal.
Looks like this in binary.
In other words only the last bit is set on 0 in decimal
looks as follows.
This is what our subnet mask looks like
255 255 255 is zero.
So in other words this portion is the network portion.
This portion is the host portion a subnet mask indicates which portion is network and which portion
is host.
So this means that the first octet is network, second octet is network, third octet is network, fourth
octet is host.
So 192.168.1.0 slash 24 means that this is network, this is host.
OK so we've been allocated this network.
How do we subnet this.
You need to decide whether you're gonna use host or network to do your subnetting.
In this example we're gonna look at hosts.
We've got a requirement for 60 hosts on two subnets, when you want to work out the number of bits required
to support a certain number of hosts.
Use the formula 2 to the power of x where x is the number of bits that is required for hosts minus 2.
If you want to work out the number of networks you use the formula 2 to the power of x where x is the
number of bits that you're gonna be using for networks.
The reason why we have to subtract two here is we need to we, need to allocate two addresses for special
use cases.
One is network or subnet and one is the broadcast address so hence we need to subtract two.
So if we're gonna support 60 hosts. That's what we've got in our topology here.
We've got two subnets consisting of 60 hosts how many bits do we need.
So to work this out.
Work out how many binary bits you need.
So two to the power of two as an example minus two gives you two.
So if you use two binary bits per our formula that'll only support two hosts, OK so how many bits are required
to support 60 hosts.
The answer is 6 bits.
And that's because two to the power of six equals 64 minus two it gives you 62.
So we will have enough hosts to support 60 hosts if we use six bits.
So 60 hosts will require six bits for the host portion of the address.
Now currently 192 168 1.0 slash 24 means we have 8 bits in the host portion.
So if we need six bits in the host portion that means that two bits can be stolen if you like for subnets.
So think of subnets as stolen bits, we're gonna steal two bits from the host portion to create additional
subnets.
That means that our subnet mask changes to slash 26.
The reason why it's slash 26 is we have eight bits in the first octet for network, plus 8 bits in the second
octet, Plus 8 bits in a third octet that gives us 24.
But we also have an additional 2 bits in the fourth octet which gives us 26.
So a subnet mask in dotted-decimal notation will be 255 255 255 192 it's 192 because 128 plus
64 gives you 192 so this is our new subnet mask.
So how do we work out the different subnets.
The first subnet will be exactly this 192.168.1.0 slash 26 now to work out the next subnet look at
your subnet portion.
So what I'll do is I'll change this to another color
lets say orange.
So the orange portion or subnet portion needs to change.
This is network, this is subnet.
This is host portion, so to work out the next subnet you cycle through all the variations of binary
for the number of bits that you've got.
So think about this as two cables, you've got two cables and you can put electricity on one cable or two
cables or no cables.
So this means there's no electricity on either cable.
This means that you're putting electricity on the second cable but not the first cable.
This means that you're putting electricity on the first cable but not the second cable.
And this means that you're putting electricity on both cables.
So this is no electricity on both cables.
This is electricity
on the second cable this is electricity on the first cable.
and this is electricity on both cables.
So what does that equate to this means zero in decimal.
This however looking at the entire octet isn't 0, 0.1 in binary means that the second bit is set on.
So what that means is that this is 64.
What does this whole octet equate to?
That's 128 first binary
bit in octet is on.
What does this equate to?
This is 192.
So what we've got now are four subnets.
192.168.1.0 slash 26, 192 168 1 64 slash 26, 128 and 192.
So what we can do now is allocate the first subnet to the hosts on the left and then as an example we
could allocate the second subnet to the hosts on the right.
And then what we could do is allocate the third subnet for the serial link and that means we've got
one additional subnet for future use.
Now we can optimize the subnet and I'll show you how to do that later but for now we'll allocate a slash
26 subnet to the serial link.
Generally you don't want to do that.
You wanna allocate a slash 30 which means that you only have two IP addresses in a subnet and you
do that to conserve IP addresses.
But for now let's configure the network using slash 26 and then I'll show you later how to subnet this
further.
I hope you found this video useful if it was of benefit to you.
Please LIKE IT AND PLEASE SUBSCRIBE TO MY YOUTUBE CHANNEL.
I wish you all the very best.