This is one of multiple IP addressing and submitting scenarios rather than just working out the subnets
for a scenario.
We're going to 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 1 9 2 1 6 8 1.0 slice 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 they are
60 hosts in San Francisco and 60 hosts in New York.
OK so this is our subnet 1 9 2 1 6 8 1.0 slice 24 now slushed 24 means that there are 24 binary ones
in the subnet mask in the first octet.
We have 2 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 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 the network and which portion
is host.
So this means that the first optative network second octet is network a third octet is network fourth
octet is.
So 1 on 2 1 6 8 1.0 is 24 means that this is network as host.
OK so we've been allocated this network.
How do we submit this.
You need to decide whether you going to use host or network you are submitting.
In this example we can 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 are going to 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 Sublette and one is the broadcast address so hence we need to subtract two.
So if we're going to support 60 hosts That's what we've got enough typology here.
We've got two subnets consisting of 60 hosts how many do we need.
So to work this out.
Work out how many binary butts 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 formula that any support two hosts OK so how many bits are required
to support 60 hosts.
The answer is 6 butts.
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 plus.
So 60 hosts will require six bucks for the host portion of the address.
Now currently 1 2 1 6 8 1.0 slice 24 means we have 8 butts in the host portion.
So if we need six butts in the host portion that means that two bits can be stolen if you like for subnets.
So think of subnets as stolen but we are going to 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 like 26 is we have eight 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 bats in the fourth octet which gives us 26.
So a subnet mask and dotted decimal notation will be 2 4 5 2 4 5 2 4 5 192 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 the 1 2 1 6 8 1.0 slushed 26 not 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 a network this subnet.
This is hoost 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 the is 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 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 is 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.
But Ted is on.
What does this equate to.
This is one hundred ninety two.
So what we've got now are four subnets.
1 9 2 1 6 8 1.0 26 1 9 2 1 6 8 1 64 slice 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 want to allocate to 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.
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