Original subtitles

84% Now that we�ve explain the Why for subnetting, let�s look at the how.

84% I believe the best way to do the Subnetting is to use the binary method

84% with some of the extra shortcuts that I�m gonna show you in following slides.

84% The binary method ensures that you can subnet any scenario

84% thrown at you and extra shortcuts make it quicker and easier to do the subnetting.

84% It�s important to understand that subnetting involves stealing or

84% taking bits away from the host portion of address and then allocating those

84% stolen bits if you like to the network portion of a new address

84% so we are taking bits away from the host portion and allocating those bits to the 10

84% network portion to allow us to create multiple or new subnet out of a single subnet. 11

84% There are 2 important rules that you need to remember 12

84% when asked for the number of hosts use the formula, host equals 2 to the n - 2 13

84% I�ve already demonstrated that formula in the previous slide 14

84% but it�s important to remember that when looking for hosts use this formula 15

84% 2 to the n - 2, and note please that you count the host bits from right to left 16

84% so we'll gonna be stealing bits from the host portion allocating those bits 17

84% to the network portion but to work out the number of hosts that can be supported on 18

84% the subnet you count the number of bits from the right hand side to 19

84% the left hand side, When asked to work out the subnets for a specific 20

84% number of networks or subnets, use the formula 2 to the n and you count the network 21

84% bits from the left hand side to the right hand side. 22

84% A lot of people get really confused by this. 23

84% Please remember the formula for host is 2 to the n - 2 24

84% The reason why we deduct 2 is that an address needs to be allocated for the subnet 25

84% and another address has to be allocated for the broadcast address. 26

84% So you lost 2 addresses or remaining addresses can be used 27

84% as addresses applied to hosts. 28

84% When working out the number of networks, you do not subtract 2 29

84% you are working out networks or subnets rather than host addresses 30

84% so you do not deduct 2 from the formula. 31

84% Also remember when as for host you count from the right to the left 32

84% when ask for network you count from the left to the right. 33

84% If you don�t follow this procedure you�re answers will be wrong. 34

84% Now this is another shortcut table that I would write down 35

84% before starting my exam to save on time. 36

84% In the real world we obviously have calculators to help us but in the exam 37

84% there are no calculators so you�ve got a work it out in your head. 38

84% In tables like this will save you a lot of time and effort 39

84% 2 to the power of 1 equals 2, 2 to the power of 2 equals 4 40

84% 2 to the power of 3 equals 8, 2 to the power of 4 equals 16 41

84% 2 to the power of 5 equals 32 and so forth and so on 42

84% until we get to 2 to the power of 10 which equals 1024. 43

84% Now you might not able to work this out quite comfortably but for a sanity check 44

84% I would suggest that you write it down before starting your exam. 45

84% Ok here's an example, let say ABC limited has been allocated 46

84% the subnet of 10.1.1.0/24 for a small office in London 47

84% Paul the networkadministrator needs to split this subnet into smaller subnets 48

84% where ach subnets support 14 host or machines. 49

84% He asked you to help him because he is not quite sure how to do subnetting. 50

84% So that will be an example of a scenario that we need to solve. 51

84% The first thing you need to decide is which formula you will gonna use. 52

84% 2 the power of n or 2 to the power of n - 2, now because the question is asking 53

84% for the number of hosts or machines in this example. 54

84% we use the formula 2 to the n - 2 and we also remember 55

84% to count from the right hand side to the left hand side. 56

84% So in step 2 we need to work out how many bits are required to cover 57

84% the number of host in this example or networks in other example. 58

84% So we have been ask to support 14 host or 14 machines as seen here in the question. 59

84% To work out the number of bits that are required you can refer back to this 60

84% table so 2 to the power of 4 equals 16. 61

84% Now remember, we are using the formula 2 to the n - 2 because we have been asked 62

84% for hosts, so 2 to the power of 4 - 2 equals 16 - 2 which gives us a result of 14 63

84% So 4 binary bits will allow us to support 14 hosts per subnet. 64

84% So we now know that we need to steal 4 bits 65

84% from the host portion of the address and allocate that for the 66

84% network portion to create multiple subnets each which support 14 hosts. 67

84% So in step 3 what you need to do is convert the host portion 68

84% of the original network into binary. 69

84% We were given network 10.1.1.0/24 or it could be written as 10.1.1.0 70

84% with the mask of 255.255.255.0, now the first 3 octets is network and the last 71

84% octet is host base on that subnet mask, please refer to the IP addressing section 72

84% of this course if your not sure how to determine 73

84% which part of the address is network and which part is host. 74

84% So we are only gonna convert the host portion of the address into binary. 75

84% For speed and efficiency purposes there is no need to convert 76

84% the network portion of the address into binary 77

84% so our address will look as follows: 10.1.1.8 binary 0's 78

84% So the host portion is 8 bits and we only need 4 bits 79

84% in the host portion to support 14 hosts. 80

84% So counting from the right hand side we count 4 binary bits which takes us 81

84% to this point and we can draw a line down to differentiate between 82

84% the subnet portion and the host portion, please note a line 83

84% can also be drawn to the right of the network portion, this was 84

84% the original subnet that we were given so we cannot manipulate this portion. 85

84% We can only manipulate the host portion and allocate part of the host portion 86

84% to the subnet and part of the host portion to the new host portion. 87

84% So we are only giving 4 bits to the host portion and the remaining 4 bits 88

84% have been stolen and allocated to the subnet portion. 89

84% Think "S" in subnet equals "S" in stolen 90

84% so 4 bit�s have been allocated to the subnet portion. 91

84% The next step is to work out the new subnet mask. 92

84% To work out the subnet mask you count the number of bits 93

84% that are network or subnet ignoring the host bits. 94

84% So from the left hand side we gonna count the number of bits in the network portion 95

84% of the address and the number of bits in the subnet portion of the address. 96

84% Please note, these spaces are only there for clarity 97

84% and not normally there as part of an address. 98

84% Now remember an octet is 8 bits. 99

84% So the first octet 10 is 8 bits, the second octet which is 1 is 8 bits in length 100

84% the 3rd octet which is also 1 is also 8 bits in length 101

84% and the number of bits that we've allocated to subnet is 4. 102

84% So we have 8 in the first octet, plus 8 in the second octet 103

84% plus 8 in the third octet, plus the 4 bits allocated to subnet in the fourth octet 104

84% which gives you a total of 28 bits. So the subnet mask is 28 bits in length. 105

84% Now you could also do this backwards, the number of bits allocated 106

84% to the host portion is 4 bits, copy address and so an IPv4 address is 32 bits 107

84% in length less 4 bits gives you 28 bits for the network portion or subnet portion. 108

84% So the last step now is to work out the various subnets. 109

84% Please remember that the network portion is 10.1.1 110

84% the subnet portion is 4 bits in length and the host portion is 4 bits in length. 111

84% And we�ve drawn a line after the network portion and the line after the host portion. 112

84% The subnet mask is 28 bits in length which we've worked out from the previous step. 113

84% So the first network is equal to 10.1.1 and we fill the subnet portion with 0's 114

84% and the host portion is always filled with 0's. 115

84% So the first network is 10.1.1.0/28 or if you convert that back to dotted decimal 116

84% notation 10.1.1.0 with the mask of 255.255.255.240 now to work out 117

84% that various subnets you go through the various binary combination 118

84% for the subnet portion of the address, so as you've seen in the first network 119

84% we set the subnet portion to all 0's. 120

84% To get the second network we set the subnet portion to all 0's 121

84% except for the last bit which we set to 1. 122

84% So the second network or subnet is 10.1.1. and if you convert this octet 123

84% back in to decimal notation it's 16, so 10.1.1.16/28, just to remind you 124

84% this is a single octet even though there are lines drawn here 125

84% the green and the red portion are part of the same octet. 126

84% so we�ve got 3 binary 0's followed by binary 1 followed by 4 binary 0's. 127

84% And if you convert that binary value into decimal value it's 16. 128

84% So the 2nd network is 10.1.1.16/28 129

90% or it can be written as follows in dotted decimal notation. 130 9:23 -9:27 align:middle line:84% Now we go. 131

84% So the next combination would be 0010 which equates to 32. 132

84% Now I won't bother doing more than said the first two or three, 133

84% because you will notice the sequence here, it goes from 0 to 16 to 32 134

84% so we are going in multiple of 16, so the next one would be 48 135

84% and you�d be able to continue all the way to the last subnet. 136

84% So I will do the first 3 or 4 and then the last 1. 137

84% For the last 1 you�ll fill the subnet portion with binary 1s 138

84% so that equals to 10.1.1.240/28 or 10.1.1.240 255.255.255.240 139

84% and that�s how you work out all the subnets from the original question. 140

84% So how many host are supported in the subnet? 141

84% We'll use the formula 2 to the n - 2, and there are 4 binary bits. 142

84% So 2 to the power of 4 equals 16 minus 2 equals 14 and we�ve already seen that. 143

84% So 14 hosts are supported on every subnet. 144

84% How many subnets have we created? 145

84% We originally had 1 subnet but we�ve now broken it up into multiple subnets 146

84% the number of bits in the subnet portion is 4 and the formula to work out 147

84% the number of networks or subnet is 2 to the power of n 148

84% so 2 to the power of 4 equals 16. 149

84% We have taken 1 subnet supporting 254 hosts 150

84% and change that into 16 subnets each supporting 14 hosts.