-: In the last lesson, 2 2
we covered a lot of details about subnets. 3 3
Now, what I want to do in this lesson 4 4
is take some of these problems and go through them, 5 5
because you may experience them come test day, 6 6
sort of walk through how to tackle them. 7 7
Now, before we get to the first problem, 8 8
the first thing I want to do is put my chart back 9 9
on the screen. 10 10
Now, why am I doing this? 11 11
Well, because I already told you, 12 12
you should commit this chart to memory. 13 13
And when you get down and you sit for the exam, 14 14
the first thing they're going to do 15 15
is give you a piece of paper and a pencil. 16 16
And once they start the clock, 17 17
you can write whatever you want on that dump sheet. 18 18
And so, one of the things I put down on my dump sheet 19 19
or my note sheet, is that I put down from my memory, 20 20
this chart, and that way, 21 21
as I get subnetting questions throughout the exam, 22 22
I can go back and look at those, 23 23
and so, it makes it really easy. 24 24
So, I'm going to give it to you here, 25 25
because I'm assuming you've memorized this before the exam, 26 26
and you've already written down 27 27
as you're sitting at your computer. 28 28
All right, let's take our first question. 29 29
Let's pretend you're a network administrator 30 30
for diontraining.com. 31 31
We decided we're going to open up a small branch office 32 32
in another city. 33 33
And so, to support that, 34 34
I need a subnet for a private IP address 35 35
that's given to you, for several different networks, 36 36
based on the different sizes of the departments 37 37
in this new location. 38 38
Now, the new office 39 39
has been given the range of 10.10.10.0/24, 40 40
which tells you, you have 256 total IP's to work with. 41 41
Now, when you set up your new network, 42 42
I want you to configure separate subnets 43 43
for each of the departments that I'm going to give you. 44 44
And you're going to tell me which CIDR notation 45 45
is going to be the summary for each one in each place. 46 46
Now, I want you to resist the urge 47 47
of going big for each subnet. 48 48
I want you to tell me the minimum subnet 49 49
that would support my requirements. 50 50
Now, in my new company, 51 51
we're going to have this number of users that we have. 52 52
So, first, I have 54 new people in IT, 53 53
I have 32 new instructors, I have five people in sales, 54 54
three in administration, 55 55
and then there's going to be an unused portion that's leftover, 56 56
and you're going to give me a subnet for that as well. 57 57
Now, each of these five areas, 58 58
I need you to tell me what the CIDR notation is for it, 59 59
and that's how we're going to answer this question, 60 60
so, first, we're going to take it step by step. 61 61
Again, you've memorized your table, 62 62
so this is going to be a pretty simple problem, 63 63
because we don't have to do the two to the S, 64 64
or the two to the H minus two calculations, 65 65
because we already have all that done in our table here. 66 66
Now, we're going to round up our department numbers 67 67
into multiples of two, or factors of two. 68 68
So, it's either going to be a one, a two, a four, an eight, 69 69
a 16, a 32, 70 70
64, or 128. 71 71
Now, remember for each of these, 72 72
I told you how many clients needed to be on the network. 73 73
For example, I say I had 54 people that work in IT, 74 74
but, you need to still account for the network address, 75 75
and the broadcast address, for each of those networks. 76 76
That's always going to be your first and last IP. 77 77
A lot of students forget that, 78 78
and they mess up the problems that way. 79 79
So, if I had 54, what do I need? 80 80
I really need 56. 81 81
Now, if I move that up 82 82
to the next closest range of two to the something power, 83 83
I get up to 64. 84 84
So, if I look at IT, 85 85
I'm going to end up having 64 IP's for that subnet, 86 86
and we have to figure out the slash notation for it. 87 87
If I look at the instructors, we had 32 instructors, 88 88
and we had to get two more, which puts us at 34, 89 89
which, rounding up, would get me back up to 64, 90 90
the same as IT. 91 91
Notice that trick there, 92 92
I would expect to see something like that on the exam, 93 93
where they put, it's something, 94 94
where they tell you I need 16, or 32, 95 95
because that looks like you'd just be able 96 96
to give them that slash notation, 97 97
but if you forget that first and last, 98 98
you're going to mess that up, so don't do that, right? 99 99
Make sure you pay attention. 100 100
Now, the next one we had is sales, 101 101
and for sales, there was five people, 102 102
so you add two to that, and that gives me seven, 103 103
and now, I look in and go, 104 104
"Oh, seven is close to eight, 105 105
so that's going to be eight people." 106 106
Next, we're going to look at admin or administration. 107 107
Administration has three people, 108 108
plus two more gets me to five, 109 109
round that up, that gets me to eight as well. 110 110
And so, that's the way we do this, right? 111 111
Now, lastly, I have to calculate the unused amount. 112 112
Now, this is going to be a little bit different, 113 113
because instead of rounding up, 114 114
we're going to have to actually round down. 115 115
So, first, we're going to start 116 116
with the total number of IP's that we had, 117 117
and we were given a slash 24 subnet, 118 118
so that means we had 256 total IP's. 119 119
Then, I'm going to take away the 64 I had for IT, 120 120
and, the 64 I had for instructors, 121 121
then I'm going to take away the eight I had for sales, 122 122
and the eight I had for administration. 123 123
And what I get left is 112 left, 124 124
but, I can't round up here, 125 125
because I'm talking about what was left. 126 126
So, I have to round down to the closest power of two, 127 127
which, in our case, is going to be a 64 again. 128 128
So, while this problem originally looked really tough, 129 129
it really isn't, 130 130
because we only have to calculate two things. 131 131
We need to figure out what's the CIDR notation for 64, 132 132
because that's what we're going to use for IT, 133 133
the instructors, and unused. 134 134
And then, we had sales and administration 135 135
that each had eight, 136 136
so I need to figure out the CIDR notation for eight. 137 137
And so, if I figure those two out, it's pretty easy, 138 138
and I have my chart, 139 139
so if I look there for 64, that's a slash 26. 140 140
So, if I have slash 26, 141 141
that's going to give me 64 for the instructors, 64 for IT, 142 142
and 64 for the unused. 143 143
Then, I have sales, which had eight, 144 144
and that's going to be a slash 29. 145 145
And administration again is eight, so that's a slash 29. 146 146
As you can see, this really complex problem 147 147
became really easy, as we broke it down step-by-step. 148 148
All right, for our next question, 149 149
we're going to make it a little bit simpler. 150 150
The last one may have been something 151 151
you might see in more or less of a simulation or a PDQ. 152 152
We have to calculate lots of different things, 153 153
and then pick the right ones. 154 154
But, during the test itself, it's going to be multiple choice. 155 155
So, you're going to get a problem 156 156
that looks something more like this. 157 157
Let's say, how many assignable IP addresses exist 158 158
in the network of 172.16.1.0/27? 159 159
Now, before we answer this, let me ask you this. 160 160
Does it even matter what the IP address is? 161 161
No, it really doesn't. 162 162
The only thing that matters here is that this is a slash 27. 163 163
The IP address itself is just there to mess with you, 164 164
so, you can ignore it in this particular problem. 165 165
The question becomes how many assignable IP's 166 166
are there in a network that uses a slash 27? 167 167
Well, I'm going to have to think about that here for a second. 168 168
Oh, wait, I really don't have to think about too hard, 169 169
'cause I have my chart, right? 170 170
Is it 30, 32, 14, or 64? 171 171
Well, looking at my chart, 172 172
I know it's either going to be 32, or it's going to be 30, 173 173
because a slash 27 shows me there's 32 IP's. 174 174
Now, which one is it? 175 175
The keyword here is assignable. 176 176
The question said how many assignable IP's. 177 177
Assignable IP means that I can give it to a client. 178 178
I can assign it to a computer. 179 179
I can't give away my network ID, 180 180
and I can't give away my broadcast ID. 181 181
So, out of those 32 in the subnet, 182 182
I only have 30 that are assignable. 183 183
And so, my answer here is 30. 184 184
Now, even if you couldn't remember that, using your chart, 185 185
you'd still get it down to either of those two options, 186 186
30 or 32, and you have a 50/50 shot of guessing right. 187 187
So, even if you couldn't remember, 188 188
the fact that you have the network and broadcast ID, 189 189
which I really hope you do, 190 190
because I've been beating that into your head 191 191
a couple of times now, 192 192
you still would at least get a 50/50 shot on this. 193 193
Now, let's assume you forgot to write down your chart. 194 194
How could you calculate this using your subnetting formulas? 195 195
Well, we would do two to the H minus two. 196 196
And that's going to give you your number of usable hosts. 197 197
Since we had slash 27 as our CIDR notation, 198 198
it becomes, 199 199
32 minus 27, giving me the number of host bits, 200 200
which is five. 201 201
So, two to the number of host bits, two to the fifth, 202 202
is going to give me 32 minus two, gets me back down to 30. 203 203
Now, whether you want to do it from the formula, 204 204
which is a lot more work, or memorizing my table, 205 205
either way will get you to the right answer 206 206
of 30 usable IP's, or 30 assignable IP's. 207 207
Now, just remember, 208 208
are they asking about assignable or usable IP's? 209 209
Or, are they asking about total IP's? 210 210
Depending on how the wording is in the question, 211 211
that's going to determine, if you're going to minus two or not. 212 212
If they said, "how many total IP's exist in this network?", 213 213
the answer would have been 32, 214 214
but because they use the word assignable, 215 215
you have to take into account the network ID, 216 216
and the broadcast ID. 217 217
And this is going to make sure 218 218
that you get to the right answer. 219 219
Now, let's go ahead and look at our third practice problem. 220 220
How many assignable IP's exist in the network? 221 221
If I have a network of 192.168.1.0/28, 222 222
this is just like the last problem. 223 223
If it's slash 28, we're going to look at our chart, 224 224
and come up with 16, but they said assignable, 225 225
so we take off two, 226 226
one for the network, one for the broadcast, 227 227
and this gives us 14. 228 228
And so, our answer is 16 minus two, which gives us 14.