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In this network, we've been told to subnet 1 9 2 1 6 8 1. 0 slash 24 into 4 subnets.

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In other words, we need a subnet for site 1, site 2, this serial link and this serial link.

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Now we haven't been told to use a slash 30 mask on the serial links.

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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.  

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0 slash 24 and subnet that into 4 subnets.

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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

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network and which part is the host portion.

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Currently, this portion is the network portion.

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We can't change the network portion but we can manipulate and change the host portion of the address.

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So in other words what we can do is change this last octet.

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The reason why we can't manipulate the first three octets is because that's part of the network, 24

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essentially means that we have 24 binary ones.

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Now you don't have spaces in an octet like this.

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I'm just doing it to make it easier to read, but essentially 24 means that we have 24 ones in the subnet

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mask, so that's 24 binary ones which essentially equates to 255 255 255 0.

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It's important that you know how to convert decimal to binary, this binary value equates to decimal

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255 so the host portion of the address is once again that portion.

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This portion is the network portion we can't manipulate the network portion but to the host portion

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consisting of eight binary zeros can be manipulated.

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Now when we subnet, think of subnetting kind of like stealing bits we're going to steal bits from the

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host portion.

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We need to work out how many bits are required for the number of subnets that we need, in this example

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we need four subnets.

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Now, when it comes to subnetting there are two formulas that you need to know.

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2 to the power of N and 2 to the power of N minus 2.

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This formula is used when we ask for subnets which is what we've been asked in this question.

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This formula we need to use when we're asked for a certain number of hosts.

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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

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each having 4 hosts we would use 2 to the power of N where N is the number of bits that are required

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minus 2.

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So if we needed to get 4 hosts we would actually need 2 to the power of 3 minus 2 which would

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be 2 to the power of 3, which is 8 minus 2 which means that we would be able to have 6 hosts

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per subnet.

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We need to subtract 2 because of broadcast and network address but we don't need that.

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When asked for networks, 2 to the power of 2 equals 4 which is what we need for the number of

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subnets that we've been asked to create.

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This is the number of bits that we're stealing or using for the subnet portion.

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This is the number of bits that we would keep for the host portion.

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Now, we're not using that formula so I can basically scratch that from our example.

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So basically remove that, we are not using that formula.

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In this example, this is the formula that we need.

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And once again that's because we've been asked 4 subnets we haven't been asked for hosts.

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Now, 2 to the power of 1 equals 2, that doesn't give us enough subnets, 2 to the power of 2 gives

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us 4 subnets, 2 to the power of 3 gives us 8 subnets, 2 to the power of 4 would give

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us 16 subnets, we don't need that many we only need 4 subnets.

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So all we need to do is use 2 bits which means we can steal 2 bits from the host portion for our

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subnet portion.

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So the host portion is now only 6 bits in length, the subnet portion is now 2 bits in length.

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So I'll write it like that.

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Once again there's obviously no spaces here in binary but that hopefully just makes it easier to read.

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So we're going to steal 2 bits from the host portion and allocate that to the subnet portion.

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So how many bits are part of the network and subnet.

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We've got our 8 bits from our original example.

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An additional 8 bits from our original example plus 8 bits.

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So that's 16 plus 1 plus 1.

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So, in other words, this is now a slash 26 subnet it's no longer slash 24 because we've stolen

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bits from the host portion for the network portion.

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So the first network that we have is 1 9 2 1 6 8 1. 0.

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Look at these 8 binary bits to make it easier to read.

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Notice we've got 8 binary bits, I've just split them up with spaces here to make it easier to demonstrate

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which part is subnet, which part is host.

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But notice there are 8 binary bits there, 8 binary bits equates to 0 in decimal.

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So the network is 1 9 2 1 6 8 1. 0 slash 26.

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That's our first network.

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Now what's the second network.

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The second network is 1 9 2 1 6 8 1

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dot and what we do now is we just go through the different binary options.

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This is 0 0 this is 0 1.

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This one would be 1 0 and this would be 1 1.

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So those are the different binary options that we have.

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So if we'll look at this network that second binary bit is set to a 1, what does this equal to, this

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equals to 64 in a decimal.

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So the next network is 64.

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What is this equal.

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It looks like this.

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Which equals 128.

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Now, as soon as you've worked out this second subnet you can simply do addition by that number.

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So 0 plus 64, 64 plus 64 is 128 plus 64 is 192.

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But if you look at the binary it's 128 plus 64, which equals 192.

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So there are our 4 subnets that we've been asked to work out in this example.

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So we've got 4 subnets the first one is for site 1, the second one is for the link between router

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1, an Internet router, third one is for site 2, fourth one is for the link between router 2 and

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the Internet router, so the first subnet would be 1 9 2 1 6 8 1. 0 slash 26.

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And I'll zoom in here to make it clearer.

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So that's our first subnet

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per our calculations, second subnet is 1 9 2 1 6 8 1 64 slash 26.

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Third one is 1 9 2 1 6 8 1 128 slash 26.

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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

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so we need to now configure the network with this information in the first step we need to work out

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the subnets but now we need to configure the routers per the instructions so as an example the last

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IP in the subnet should be configured on router 1, second last IP address should be configured on the

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switch, third last IP address should be configured on the DHCP server per these instructions we also need

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to configure a DHCP pool on the DHCP server so I'm going to start with the subnet first and get that

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working and then I'll move to the other subnets.