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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.
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Now we go.
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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.
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