In this network, we've been told to subnet 1 9 2 1 6 8 1. 0 slash 24 into 4 subnets.
In other words, we need a subnet for site 1, site 2, this serial link and this serial link.
Now we haven't been told to use a slash 30 mask on the serial links.
We'll do that in a separate video so all we need to do at this point is take 1 9 2 1 6 8 1.
0 slash 24 and subnet that into 4 subnets.
Now when we look at a network address such as 1 9 2 1 6 8 1. 0 we need to determine which part is the
network and which part is the host portion.
Currently, this portion is the network portion.
We can't change the network portion but we can manipulate and change the host portion of the address.
So in other words what we can do is change this last octet.
The reason why we can't manipulate the first three octets is because that's part of the network, 24
essentially means that we have 24 binary ones.
Now you don't have spaces in an octet like this.
I'm just doing it to make it easier to read, but essentially 24 means that we have 24 ones in the subnet
mask, so that's 24 binary ones which essentially equates to 255 255 255 0.
It's important that you know how to convert decimal to binary, this binary value equates to decimal
255 so the host portion of the address is once again that portion.
This portion is the network portion we can't manipulate the network portion but to the host portion
consisting of eight binary zeros can be manipulated.
Now when we subnet, think of subnetting kind of like stealing bits we're going to steal bits from the
host portion.
We need to work out how many bits are required for the number of subnets that we need, in this example
we need four subnets.
Now, when it comes to subnetting there are two formulas that you need to know.
2 to the power of N and 2 to the power of N minus 2.
This formula is used when we ask for subnets which is what we've been asked in this question.
This formula we need to use when we're asked for a certain number of hosts.
So if we were asked to subnet this subnet 1 9 2 1 6 8 1. 0 slash 24 and create as many subnets as possible
each having 4 hosts we would use 2 to the power of N where N is the number of bits that are required
minus 2.
So if we needed to get 4 hosts we would actually need 2 to the power of 3 minus 2 which would
be 2 to the power of 3, which is 8 minus 2 which means that we would be able to have 6 hosts
per subnet.
We need to subtract 2 because of broadcast and network address but we don't need that.
When asked for networks, 2 to the power of 2 equals 4 which is what we need for the number of
subnets that we've been asked to create.
This is the number of bits that we're stealing or using for the subnet portion.
This is the number of bits that we would keep for the host portion.
Now, we're not using that formula so I can basically scratch that from our example.
So basically remove that, we are not using that formula.
In this example, this is the formula that we need.
And once again that's because we've been asked 4 subnets we haven't been asked for hosts.
Now, 2 to the power of 1 equals 2, that doesn't give us enough subnets, 2 to the power of 2 gives
us 4 subnets, 2 to the power of 3 gives us 8 subnets, 2 to the power of 4 would give
us 16 subnets, we don't need that many we only need 4 subnets.
So all we need to do is use 2 bits which means we can steal 2 bits from the host portion for our
subnet portion.
So the host portion is now only 6 bits in length, the subnet portion is now 2 bits in length.
So I'll write it like that.
Once again there's obviously no spaces here in binary but that hopefully just makes it easier to read.
So we're going to steal 2 bits from the host portion and allocate that to the subnet portion.
So how many bits are part of the network and subnet.
We've got our 8 bits from our original example.
An additional 8 bits from our original example plus 8 bits.
So that's 16 plus 1 plus 1.
So, in other words, this is now a slash 26 subnet it's no longer slash 24 because we've stolen
bits from the host portion for the network portion.
So the first network that we have is 1 9 2 1 6 8 1. 0.
Look at these 8 binary bits to make it easier to read.
Notice we've got 8 binary bits, I've just split them up with spaces here to make it easier to demonstrate
which part is subnet, which part is host.
But notice there are 8 binary bits there, 8 binary bits equates to 0 in decimal.
So the network is 1 9 2 1 6 8 1. 0 slash 26.
That's our first network.
Now what's the second network.
The second network is 1 9 2 1 6 8 1
dot and what we do now is we just go through the different binary options.
This is 0 0 this is 0 1.
This one would be 1 0 and this would be 1 1.
So those are the different binary options that we have.
So if we'll look at this network that second binary bit is set to a 1, what does this equal to, this
equals to 64 in a decimal.
So the next network is 64.
What is this equal.
It looks like this.
Which equals 128.
Now, as soon as you've worked out this second subnet you can simply do addition by that number.
So 0 plus 64, 64 plus 64 is 128 plus 64 is 192.
But if you look at the binary it's 128 plus 64, which equals 192.
So there are our 4 subnets that we've been asked to work out in this example.
So we've got 4 subnets the first one is for site 1, the second one is for the link between router
1, an Internet router, third one is for site 2, fourth one is for the link between router 2 and
the Internet router, so the first subnet would be 1 9 2 1 6 8 1. 0 slash 26.
And I'll zoom in here to make it clearer.
So that's our first subnet
per our calculations, second subnet is 1 9 2 1 6 8 1 64 slash 26.
Third one is 1 9 2 1 6 8 1 128 slash 26.
And the last one is 1 9 2 1 6 8 1 192 slash 26 so those subnets are the subnets that we worked out
so we need to now configure the network with this information in the first step we need to work out
the subnets but now we need to configure the routers per the instructions so as an example the last
IP in the subnet should be configured on router 1, second last IP address should be configured on the
switch, third last IP address should be configured on the DHCP server per these instructions we also need
to configure a DHCP pool on the DHCP server so I'm going to start with the subnet first and get that
working and then I'll move to the other subnets.