Original subtitles

In this section we gonna look at IP subnetting.

I�d like to teach you multiple methods for subnetting.

Subnetting is one of those subject that a lot of Network Engineers struggle with.

But in this section I hope I'll be able to simplify for you

and give you various methods.

So that any subnetting questions that you might encounter

can be completed quickly and easily.

There are 2 main parts in this section:

in the 1st part I'm gonna show you 2 methods to quickly and easily answer 10

questions like, what subnet is this host on or what is a last host in this subnet? 11

So I�m gonna teach you the binary method and then 12

I�m gonna teach you what I call the quick method. 13

I suggest that you look at both and then choose one 14

and get good at that specific method. 15

I think binary is very important to understand. 16

But as the quick method name indicates it�s very quick and easy 17

to answer subnetting questions using the quick method. 18

In the 2nd part I�d like to show you how to create multiple subnets 19

when given a specific network or subnet, so you will be able to answer questions 20

like subnet this network into at least 10 subnets or subnet this network 21

into subnets each having 10 hosts on them. 22

So in part 1 you'll be given an IP address and then you gonna need to learn 23

how to work out the subnet address, 1st host address, last host address 24

and the broadcast address and we'll gonna use 2 methods: the binary method 25

and the quick method, and as I mentioned I suggest that you choose 1 of the 26

2 methods, look at both, decide which one you prefer 27

and then learn and get good at that specific method. 28

So here's the typical example: 29

You�ve been given a host with an address of 192.168.10.18 for the /24 mask 30

and you�ll need to be able to answer the following questions: 31

what IP address with router 1 be configured with, this router being router 1 32

if it is to use the first IP address in the same subnet as PC 1? 33

Which is the PC here? The second question could be: 34

What broadcast address is used by PC 1? 35

or thirdly what IP address would router 1 be configured with if it is to use 36

the last IP address in the same subnet as PC1? 37

So the first question was, what IP address will the router use if it is 38

configured with the first IP address? 39

And the third one is: what IP address would it be configured with if it were 40

configured with the last IP address in the subnet? 41

And fourthly, a question could be what subnet is PC 1 part of? 42

so you should within 30 to 60 seconds be able to answer those question 43

and I�m gonna show you how to do that now. 44

So the first method is to use binary to work out the answers. 45

I personally prefer binary because it's used all over the place in networking. 46

And I want you get your head around how binary works. 47

It will save you a lot of time in the long run. 48

Please refer to the binary section of this course 49

if you are not sure about the basics of binary. 50

we are going to assume knowledge of binary now 51

we're just gonna use binary to work out the answer to those questions. 52

In the binary method, you need to remember 4 rules 53

to work out the network with subnet address 54

you fill the host portion of the address with binary 0's 55

So you work out which part is network and which part is host 56

and you fill the host portion with binary 0's. 57

To work out the broadcast address, 58

you fill the host portion of the address with binary 1's. 59

To work out the first host in the subnet, you fill the host portion of the 60

address with binary 0's except for the very last bit which is set to a binary 1. 61

And to work out the last host in the subnet, you fill the host portion of the 62

address with binary 1's except for the last bit which is set to binary 0. 63

If you remember those 4 rules, you will be able to work out the answer to any 64

of those questions very quickly. 65

So let�s start the basic example. 66

We have a PC with an IP address 192.168.1.18/24 or it could be written as 67

192.168.1.18 with the mask of 255.255.255.0 68

As you learnt in the IP addressing section, these 2 are equivalent subnet mask. 69

It's very similar to saying tomato or tomato or router or router. 70

It means the same thing it's just a different way of displaying the subnet mask. 71

Now hopefully you remember that 24 means 24 bits in the subnet mask 72

in other word 8 bits in the first octet, 8 bits in the second octet 73

and the 8 bits in the third octet. That means the first 3 octets are network. 74

So 192.168.1 is the network portion of the address and this last octet is the 75

host portion and that�s how I�ve written it here. 76

The first 3 octets are displayed in black representing the network portion and 77

the last octet is shown in red which represents the host portion of the address. 78

There�s no point converting the network portion into binary. 79

So I�ve just left as it is, so the subnet is equal to 192.168.1. 80

and if you remember back to our rules to get the subnet, 81

you fill the host portion of the address with binary 0's 82

So this is the single octet, so 8 binary 0's need to fill that host portion. 83

If you convert 8 binary 0's back into decimal you'll get the value 0. 84

So the subnet is 192.168.1.0 for this specific IP address. 85

To get the first host in the subnet, you fill the host portion of the address 86

with binary 0's except for the last bit which make binary 1. 87

So an octet once again is 8 bits. 88

So there are 7 binary 0's followed by single binary 1. 89

Converting that back to decimal you get 1, 90

so the first host in this subnet is 192.168.1.1 91

To work out the last host, fill the host portion of the address with binary 1's 92

except for the last bit you set to binary 0 93

That's 7 binary 1's followed by binary 0, if you convert that back to decimal 94

you'll get the value 254. 95

So the very last host in this subnet is 192.168.1.254 96

To get the broadcast address, fill the host portion of the address 97

with binary 1's, so single octet fill with binary 1's is equal to 255 in decimal 98

So there you have it. We can work out the subnet, the first host, the last host 99

and the broadcast address very quickly and easily using the binary method. 100

So if you were ask to configure the 1st IP address in the same subnet as PC 1 101

on the routers interface you would know that you need to configure the IP address 102

192.168.1.1/24 or 192.168.1.1 255.255.255.0 103

You could be expected to type the commands unto the router to configure it with 104

the relevant IP address, later in this course you'll learn the commands to do that 105

But for now, realize that you might need to work out first, last, broadcast and 106

subnet addresses When giving a specific IP address. 107

This is our first example using the binary method 108

I�d like to show you more complicated example now. 109

So in this example you've been given an IP address 172.16.35.123/20 110

or in other words 255.255.240.0 111

This is a little bit more complicated because the mask 112

does not fall on the octet boundary, 113

in other words, it's not a simple /8 or /16 or /24 subnet. 114

The network and host portion lies somewhere in the third octet. 115

So just to reiterate, /20 means that 20 bits of the 32 bit IP address 116

which are used for networks or subnet portion and the remaining 12 bits are used 117

as the host portion of the address. 118

So somewhere in the third octet, the network host portion changes. 119

So once again our IP address is 172.16.35.123/20 120

Now the first octet is 8 bits in size , the second octet is 8 bits in size. 121

So adding those together you get 16 bits. 122

The third octet is 8 bits in size, so adding those 3 together gives you 24 bits. 123

So /20 in the subnet mask means that the split between subnet 124

and host occurs somewhere in the third octet. 125

So once again the first octet is 8 bits, second octet is 8 bits thus 126

20 bits puts the split somewhere in the third octet. 127

So what you need to do is convert the third and fourth octet 128

into binary because they both have host bits. 129

There�s no need to convert the first 2 octets into binary 130

because no host bits are found in the first 2 octets. 131

So 00100011 is the binary equivalent of decimal 35, 0 followed by 4 binary 1's 132

followed by 0 followed by 2 binary 1's is the binary equivalent of decimal a 123. 133

So all we�ve done is convert 35 and 123 into their binary equivalents. 134

Please see the binary section of this course if you are not 135

sure how to convert decimal into binary and back again. 136

20 bits of subnet mask put us in this position because we have 8 bits 137

plus 8 bits in the first 2 octets which give us a total 138

of 16 binary bits plus 1 is 17 plus 1 is 18 plus 1 is 19 plus 1 is 20 139

So we can draw a line at this point to split the network and host portion 140

To the left of this line, the binary bits indicate network or subnet, 141

to the right of this line the binary bits indicates host. 142

So there are split between network and host portions in this address at this line. 143

So to work out the answers you need to remember the 4 rules. 144

The first rule states that, to work out the network or subnet address, 145

you need to fill the host portion of the address with binary 0's. 146

So to the right of the line we need to fill 147

this entire portion of the address with binary 0's. 148

So once again 20 bits of subnet mask put us in this position 149

and we fill everything to the right with binary 0's. 150

So we end up having 172.16 the first 4 bits of the third octet 151

remain the same and all remaining bits are set to binary 0. 152

When you convert that back to decimal, please note firstly that these 4 bits 153

in red plus the following 4 bits in green are part of the third octet. 154

So 0010 followed by 0000 when converted from binary 155

to decimal gives us a value of 32. The fourth octet is filled with 0's. 156

So converting an octet filled with binary 0's to decimal give us a value of 0. 157

So the subnet address is 172.16.32.0 158

Now to work out the first host address, fill the host portion of the address 159

with binary 0's except for the last bit which is set to binary 1. 160

So here you go, the first host is equal to 172.16.0010 161

remember, we don�t touch this portion of the address, we only change the host 162

portion of the address, so we fill the host portion with binary 0's 163

except for the last bit which we set to binary 1. 164

Once again the third octet consist of 4 binary bits which are part of the network 165

or subnet and 4 binary bits which a part of the host portion. 166

Those 4 red binary bits together with those 4 green binary bits 167

give us a decimal value of 32. 168

The last octet is 7 binary 0's followed by single binary 1 169

which is equal to decimal 1, so the first host address is 172.16.32.1 170

To work out the last host address, fill the host portion of the address 171

with binary 1's except for the last bit which is set to binary 0. 172

So the last host is equal to 172.16.0010 followed 4 binary 1's, in the third octet 173

followed by 7 binary 1's in the fourth octet and then lastly a binary 0. 174

So the third octet consist of this 4 red bits and this 4 green bits. 175

So if you convert 0010 followed by 4 1's into decimal 176

you'll get the decimal equivalent which is 47, 7 binary 1's 177

followed by binary 0, gives you the equivalent decimal value of 254. 178

So the last host in this subnet is 172.16.47.254 179

To work out the broadcast address, fill the host portion 180

of the address with binary 1's. 181

So the broadcast address is equal to 172.16.0010 182

Once again, the network and subnet portion are left as is. 183

And then you fill the host portion with binary 1's. 184

So the third octet is equal to 0010 and 4 binary 1's 185

which once again gives you the decimal equivalent of 47. 186

8 binary 1's in octet gives you the decimal value 255. 187

So the broadcast address would be 172.16.47.255 188

So in summary, an address of 172.16.35.123 with the subnet mask of /20 resides 189

on subnet and has a broadcast address of 190

the first host in the same subnet is 172.16.32.1 191

and the last host in the same subnet is 172.16.47.254 192

This was a slightly more complicated example, but I�m hoping that by now 193

you have learned the process to work out subnet first host, last host and 194

broadcast address when given an IP address and its subnet mask.