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This is one of multiple IP addressing and subnetting scenarios rather than just working out the subnets

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for a scenario.

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We're gonna work them out and then configure the devices so that we can practically design and configure

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a Cisco IP network. In this scenario

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let's assume that we've got two sites one in San Francisco one in New York.

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The sites are connected via a serial link.

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In this scenario, we've been allocated a subnet of 192 168 1.0 slash 24.

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And we need to subnet this network to support this topology.

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Let's assume that this is a small medium business.

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So, all PCs at each site will be in the same subnet.

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We're going to keep it simple by configuring the hosts in a single subnet at each site but there are

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60 hosts in San Francisco and 60 hosts in New York.

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OK, so this is our subnet 192 168 1.0 slash24, now slash 24 means that there are 24 binary ones

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in the subnet mask. In the first octet

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we have eight binary ones.

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Second octet to 8 binary ones a third octet a binary ones.

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Now in an octet there is no space between the binary ones.

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I'm just going to represent them that way to make it easier to read 24 is a side notation or classless

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into domain writing notation.

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Twenty four means that the subnet mask again consists of 24 binary ones.

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Again there is no space between the binary ones.

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But to make it easier to read.

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I'm going to show it with spaces.

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So he has octet 1 8 binary ones equates to 255 and decimal 8 binary ones equates to 255 8 binary ones

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equates to 255 8 to binary zeros equates to 0 using decimal.

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So this is the dotted decimal notation of the subnet mask.

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This is the Sajda notational binary notation.

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Now when working with binary Don't forget the following.

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If you have a 1 in the least significant but in the octet that represents 1 in decimal 1 in the second

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least significant position in binary equates to 2 in decimal that equates to 4 and so forth and so 1

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1 in the most significant position in the octet equates to 128.

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So if we look at 1 9 2 1 6 8 1 2 0 in binary 1 9 2 is that.

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In other words it's 128 plus 64 168 in decimal.

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Looks like this in binary.

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In other words it's 128 plus 32 plus 8 1 in decimal.

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Looks like this in binary.

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In other words only the last bit is set on 0 in decimal

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looks as follows.

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This is what our subnet mask looks like

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255 255 255 is zero.

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So in other words this portion is the network portion.

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This portion is the host portion a subnet mask indicates which portion is network and which portion

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is host.

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So this means that the first octet is network, second octet is network, third octet is network, fourth

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octet is host.

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So 192.168.1.0 slash 24 means that this is network, this is host.

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OK so we've been allocated this network.

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How do we subnet this.

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You need to decide whether you're gonna use host or network to do your subnetting.

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In this example we're gonna look at hosts.

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We've got a requirement for 60 hosts on two subnets, when you want to work out the number of bits required

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to support a certain number of hosts.

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Use the formula 2 to the power of x where x is the number of bits that is required for hosts minus 2.

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If you want to work out the number of networks you use the formula 2 to the power of x where x is the

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number of bits that you're gonna be using for networks.

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The reason why we have to subtract two here is we need to we, need to allocate two addresses for special

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use cases.

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One is network or subnet and one is the broadcast address so hence we need to subtract two.

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So if we're gonna support 60 hosts. That's what we've got in our topology here.

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We've got two subnets consisting of 60 hosts how many bits do we need.

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So to work this out.

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Work out how many binary bits you need.

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So two to the power of two as an example minus two gives you two.

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So if you use two binary bits per our formula that'll only support two hosts, OK so how many bits are required

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to support 60 hosts.

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The answer is 6 bits.

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And that's because two to the power of six equals 64 minus two it gives you 62.

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So we will have enough hosts to support 60 hosts if we use six bits. 

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So 60 hosts will require six bits for the host portion of the address.

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Now currently 192 168 1.0 slash 24 means we have 8 bits in the host portion.

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So if we need six bits in the host portion that means that two bits can be stolen if you like for subnets.

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So think of subnets as stolen bits, we're gonna steal two bits from the host portion to create additional

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subnets.

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That means that our subnet mask changes to slash 26.

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The reason why it's slash 26 is we have eight bits in the first octet for network, plus 8 bits in the second

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octet, Plus 8 bits in a third octet that gives us 24.

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But we also have an additional 2 bits in the fourth octet which gives us 26.

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So a subnet mask in dotted-decimal notation will be 255 255 255 192 it's 192 because 128 plus

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64 gives you 192 so this is our new subnet mask.

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So how do we work out the different subnets.

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The first subnet will be exactly this 192.168.1.0 slash 26 now to work out the next subnet look at

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your subnet portion.

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So what I'll do is I'll change this to another color

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lets say orange.

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So the orange portion or subnet portion needs to change.

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This is network, this is subnet.

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This is host portion, so to work out the next subnet you cycle through all the variations of binary

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for the number of bits that you've got.

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So think about this as two cables, you've got two cables and you can put electricity on one cable or two

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cables or no cables.

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So this means there's no electricity on either cable.

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This means that you're putting electricity on the second cable but not the first cable.

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This means that you're putting electricity on the first cable but not the second cable.

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And this means that you're putting electricity on both cables.

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So this is no electricity on both cables.

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This is electricity

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on the second cable this is electricity on the first cable.

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and this is electricity on both cables.

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So what does that equate to this means zero in decimal.

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This however looking at the entire octet isn't 0, 0.1 in binary means that the second bit is set on.

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So what that means is that this is 64.

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What does this whole octet equate to?

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That's 128 first binary

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bit in octet is on. 

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What does this equate to?

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This is 192.

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So what we've got now are four subnets.

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192.168.1.0 slash 26, 192 168 1 64 slash 26, 128 and 192.

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So what we can do now is allocate the first subnet to the hosts on the left and then as an example we

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could allocate the second subnet to the hosts on the right.

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And then what we could do is allocate the third subnet for the serial link and that means we've got

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one additional subnet for future use.

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Now we can optimize the subnet and I'll show you how to do that later but for now we'll allocate a slash

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26 subnet to the serial link.

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Generally you don't want to do that.

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You wanna allocate a slash 30 which means that you only have two IP addresses in a subnet and you

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do that to conserve IP addresses.

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But for now let's configure the network using slash 26 and then I'll show you later how to subnet this

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further.

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I hope you found this video useful if it was of benefit to you.

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