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Now that we’ve explain the Why for subnetting, let’s look at the how.

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I believe the best way to do the Subnetting is to use the binary method

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with some of the extra shortcuts that I’m gonna show you in following slides.

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The binary method ensures that you can subnet any scenario

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thrown at you and extra shortcuts make it quicker and easier to do the subnetting.

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It’s important to understand that subnetting involves stealing or

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taking bits away from the host portion of address and then allocating those

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stolen bits if you like to the network portion of a new address

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so we are taking bits away from the host portion and allocating those bits to the

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network portion to allow us to create multiple or new subnet out of a single subnet.

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There are 2 important rules that you need to remember

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when asked for the number of hosts use the formula, host equals 2 to the n - 2

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I’ve already demonstrated that formula in the previous slide

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but it’s important to remember that when looking for hosts use this formula

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2 to the n - 2, and note please that you count the host bits from right to left

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so we'll gonna be stealing bits from the host portion allocating those bits

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to the network portion but to work out the number of hosts that can be supported on

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the subnet you count the number of bits from the right hand side to

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the left hand side, When asked to work out the subnets for a specific

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number of networks or subnets, use the formula 2 to the n and you count the network

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bits from the left hand side to the right hand  side.

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A lot of people get really confused by this.

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Please remember the formula for host is 2 to the n - 2

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The reason why we deduct 2 is that an address needs to be allocated for the subnet

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and another address has to be allocated for the broadcast address.

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So you lost 2 addresses or remaining addresses can be used

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as addresses applied to hosts.

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When working out the number of networks, you do not subtract 2

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you are working out networks or subnets rather than host addresses

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so you do not deduct 2 from the formula.

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Also  remember when as for host you count from the right to the left

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when ask for network you count from the left to the right.

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If you don’t follow this procedure you’re answers will be wrong.

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Now this is another shortcut table that I would write down

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before starting my exam to save on time.

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In the real world we obviously have calculators to help us but in the exam

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there are no calculators so you’ve got a work it out in your head.

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In tables like this will save you a lot of time and effort

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2 to the power of 1 equals 2, 2 to the power of 2 equals 4

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2 to the power of 3 equals 8, 2 to the power of 4 equals 16

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2 to the power of 5 equals 32 and so  forth and so on

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until we get to 2 to the power of 10 which equals 1024.

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Now you might not able to work this out quite comfortably but for a sanity check

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I would suggest that you write it down before starting your exam.

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Ok here's an example, let say ABC limited has been allocated

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the subnet of 10.1.1.0/24 for a small office in London

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Paul the networkadministrator needs to split this subnet into smaller subnets

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where ach subnets support 14 host or machines.

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He asked you to help him because he is not quite sure how to do subnetting.

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So that will be an example of a scenario that we need to solve.

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The first thing you need to decide is which formula you will gonna use.

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2 the power of n or 2 to the power of n - 2, now because the question is asking

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for the number of hosts or machines in this example.

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we use the formula 2 to the n - 2 and we also remember

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to  count from the right hand side to  the left hand side.

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So in step 2 we need to work out how many bits  are required to cover

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the number of host in this example or networks in other example.

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So we have been ask to support 14 host or 14 machines as seen here in the question.

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To work  out the number of bits that are required you can refer back to this

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table so 2 to the power of 4 equals 16.

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Now remember, we are using the  formula 2 to the n - 2 because we have been asked

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for hosts, so 2 to the power of 4 - 2 equals 16 - 2 which gives us a result of 14

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So 4 binary  bits will allow us to support 14 hosts per subnet.

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So we now know that we  need to steal 4 bits

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from the host portion of the address and allocate that for the

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network portion to create multiple subnets each which support 14 hosts.

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So in step 3 what you need to do is convert the host portion

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of the original network into binary.

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We were given network 10.1.1.0/24 or it could be written as 10.1.1.0

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with the mask of 255.255.255.0, now the first 3 octets is network and the last

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octet is host base on that subnet mask, please refer to the IP addressing section

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of this course if your not sure  how to determine

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which part of the address is network and which part is host.

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So we are only gonna convert the host portion of the address into binary.

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For speed and efficiency purposes there is no need to convert

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the network portion of the address into binary

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so our address will look as follows: 10.1.1.8 binary 0's

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So the host portion is 8 bits and we only need 4 bits

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in the host portion to support 14 hosts.

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So counting from the right hand side we count 4 binary bits which takes us

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to this point and we can draw a line down to differentiate between

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the subnet portion and the host  portion, please note a line

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can also be drawn to the right of the network portion, this was

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the original subnet that we were given so we cannot manipulate this portion.

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We can only manipulate the host portion and allocate part of the host portion

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to the subnet and part of the host portion to the new host portion.

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So we are only giving 4 bits to the host portion and the  remaining 4 bits

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have been stolen and allocated to the subnet portion.

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Think "S" in subnet equals "S" in stolen

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so 4 bit’s have been allocated to the subnet portion.

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The next step is to work out the new subnet mask.

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To work out the subnet mask you count  the number of bits

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that are network or subnet ignoring the host bits.

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So from the left hand side we gonna count the number of bits in the  network portion

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of the address and  the number of bits in the subnet portion of the address.

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Please note, these spaces are only there for clarity

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and not normally there as  part of an address.

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Now remember an octet is 8 bits.

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So the first octet 10 is 8 bits, the second  octet which is 1 is 8 bits in length

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the 3rd octet which is also 1 is also 8 bits in length

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and the number of bits that we've allocated to subnet is 4.

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So we have 8 in the first octet, plus 8 in the second octet

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plus 8 in the third octet, plus the 4 bits allocated to subnet in the fourth octet

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which gives you a total of 28 bits. So the subnet mask is 28 bits in length.

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Now you could also do this backwards, the number of bits allocated

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to the host portion is 4 bits, copy address and so an IPv4 address is 32 bits

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in length less 4 bits gives you 28 bits for the network portion or subnet portion.

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So the last step now is to work out the various subnets.

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Please remember that the network portion is 10.1.1

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the subnet portion is 4 bits in length and the host portion is 4 bits in length.

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And we’ve drawn a line after the network portion and the line after the host portion.

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The subnet mask is 28 bits in length which we've worked out from the previous step.

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So the first network is equal to 10.1.1 and we fill the subnet portion with 0's

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and the host portion is always filled with 0's.

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So the first network is 10.1.1.0/28 or if you convert that back to dotted decimal

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notation 10.1.1.0 with the mask of 255.255.255.240 now to work out

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that various subnets you go through the various binary combination

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for the subnet portion of the address, so as you've seen in the first network

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we set the subnet portion to all 0's.

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To get the second network we set the subnet portion to all 0's

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except for the last bit which we set to 1.

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So the second network or subnet is 10.1.1. and if you convert this octet

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back in to decimal notation it's 16, so 10.1.1.16/28, just to remind you

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this is a single octet even though there are lines drawn here

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the green and the red portion are part of the same octet.

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so we’ve got 3 binary 0's followed by binary 1 followed by 4 binary 0's.

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And if you convert that binary value into decimal value it's 16.

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So the 2nd network is 10.1.1.16/28

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or it can be written as follows in dotted decimal notation.
9:23 -9:27

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So the next combination would be 0010 which equates to 32.

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Now I won't bother doing more than said the first two or three,

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because you will notice the sequence here, it goes from 0 to 16 to 32

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so we are going in multiple of 16, so the next one would be 48

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and you’d be able to continue all the way to the last subnet.

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So I will do the first 3 or 4 and then the last 1.

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For the last 1 you’ll fill the subnet portion with binary 1s

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so that equals to 10.1.1.240/28 or 10.1.1.240 255.255.255.240

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and that’s how you work out all the subnets from the original question.

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So how many host are supported in the subnet?

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We'll use the formula 2 to the n - 2, and there are 4 binary bits.

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So 2 to the power of 4 equals 16 minus 2 equals 14 and we’ve already seen that.

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So 14 hosts are supported on every subnet.

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How many subnets have we created?

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We originally had 1 subnet but we’ve now broken it up into multiple subnets

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the number of bits in the subnet portion is 4 and the formula to work out

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the number of networks or subnet is 2 to the power of n

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so 2 to the power of 4 equals 16.

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We have taken 1 subnet supporting 254 hosts

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and change that into 16 subnets each supporting 14 hosts.