1 1 00:00:00,570 --> 00:00:02,940 Alright, now that we got those computer mathematics 2 2 00:00:02,940 --> 00:00:06,760 under our belt, let's dive into the concept of subnetting. 3 3 00:00:06,760 --> 00:00:08,800 Subnetting is where we can take a large network 4 4 00:00:08,800 --> 00:00:11,150 and we split it up into smaller networks. 5 5 00:00:11,150 --> 00:00:14,690 When we do this, this is all about logical IP addressing. 6 6 00:00:14,690 --> 00:00:16,550 The default classful subnet mask 7 7 00:00:16,550 --> 00:00:20,610 is rarely going to be the optimal subnet for our subnet size. 8 8 00:00:20,610 --> 00:00:24,120 So, instead, we use subnet masks to modify the subnets 9 9 00:00:24,120 --> 00:00:26,930 and create networks that are much better in scope. 10 10 00:00:26,930 --> 00:00:28,820 For instance, let's say I gave you a network 11 11 00:00:28,820 --> 00:00:31,380 of 10 dot something dot something dot something, 12 12 00:00:31,380 --> 00:00:33,220 this is a class A address, that means 13 13 00:00:33,220 --> 00:00:37,560 there are 16.7 million IP addresses in that range. 14 14 00:00:37,560 --> 00:00:39,050 You may have a large network, 15 15 00:00:39,050 --> 00:00:42,760 but your network is probably not 16.7 million clients. 16 16 00:00:42,760 --> 00:00:45,130 So, we would want to create subnets 17 17 00:00:45,130 --> 00:00:48,100 that allow us to borrow bits from the original host portion 18 18 00:00:48,100 --> 00:00:50,490 and then we can add them into the network portion 19 19 00:00:50,490 --> 00:00:52,840 to make smaller networks for us. 20 20 00:00:52,840 --> 00:00:57,550 So, for example, if I had a network like 10.0.0.0/8, 21 21 00:00:57,550 --> 00:00:59,840 that is a classful class A subnet, 22 22 00:00:59,840 --> 00:01:02,560 and it has 16.7 million hosts. 23 23 00:01:02,560 --> 00:01:04,410 But I probably need something like 24 24 00:01:04,410 --> 00:01:07,700 a couple of hundred hosts, maybe 256 of them. 25 25 00:01:07,700 --> 00:01:08,790 Well, if I wanted to do that, 26 26 00:01:08,790 --> 00:01:12,630 I might use the class C subnet mask of slash 24 27 27 00:01:12,630 --> 00:01:14,020 and use that, instead. 28 28 00:01:14,020 --> 00:01:17,270 By using 10.0.0.0/24, 29 29 00:01:17,270 --> 00:01:20,890 that's going to give me 256 IPs in that subnet, 30 30 00:01:20,890 --> 00:01:22,960 and that means, those other 16 million 31 31 00:01:22,960 --> 00:01:24,750 can be used by other people. 32 32 00:01:24,750 --> 00:01:26,630 Then, maybe I'm going to create a second subnet, 33 33 00:01:26,630 --> 00:01:31,570 like 10.0.1.0/24, and then I can create a third subnet 34 34 00:01:31,570 --> 00:01:36,570 with 10.0.2.0/24, and now I've used three subnets 35 35 00:01:36,610 --> 00:01:40,900 of 256 IPs each, and I still have millions of leftover IPs 36 36 00:01:40,900 --> 00:01:42,710 that I can subnet out later. 37 37 00:01:42,710 --> 00:01:45,040 This is why it's really important to use subnets, 38 38 00:01:45,040 --> 00:01:47,960 because it's an efficient use of the IP addresses you have, 39 39 00:01:47,960 --> 00:01:50,540 instead of just using the classful default. 40 40 00:01:50,540 --> 00:01:51,800 Even in your home network, 41 41 00:01:51,800 --> 00:01:53,440 if you're using a class C address 42 42 00:01:53,440 --> 00:01:58,310 of something like 192.168.1.0/24, 43 43 00:01:58,310 --> 00:02:00,880 this gives you 256 possibilities, 44 44 00:02:00,880 --> 00:02:02,790 but you probably only have five or 10 devices 45 45 00:02:02,790 --> 00:02:04,400 on your home network. 46 46 00:02:04,400 --> 00:02:06,530 Now, by doing subnets, this is going to allow us 47 47 00:02:06,530 --> 00:02:08,680 to create additional VLANs in subnets 48 48 00:02:08,680 --> 00:02:11,750 and allow separation of our networks for better security 49 49 00:02:11,750 --> 00:02:14,330 and giving us better bandwidth control. 50 50 00:02:14,330 --> 00:02:16,020 Here on the screen you can see a chart 51 51 00:02:16,020 --> 00:02:19,600 that has the addresses for the class A, B, and C subnets 52 52 00:02:19,600 --> 00:02:20,950 and their default masks, 53 53 00:02:20,950 --> 00:02:23,040 which should be a review for you at this point. 54 54 00:02:23,040 --> 00:02:24,810 Now, on the right side, you're going to see 55 55 00:02:24,810 --> 00:02:26,870 the assignable number of IP addresses, 56 56 00:02:26,870 --> 00:02:30,220 which is for class A, of 16.7 million addresses, 57 57 00:02:30,220 --> 00:02:33,640 for class B, 65,000 plus IP addresses, 58 58 00:02:33,640 --> 00:02:37,360 and class C, 254 usable IP addresses, 59 59 00:02:37,360 --> 00:02:39,900 that is a lot of IP addresses. 60 60 00:02:39,900 --> 00:02:41,700 Now, we may want to start nicking those down 61 61 00:02:41,700 --> 00:02:43,030 into usable chunks, 62 62 00:02:43,030 --> 00:02:45,550 and that is what we're going to use subnetting for. 63 63 00:02:45,550 --> 00:02:48,540 If we're using private IPs, this really isn't a big deal, 64 64 00:02:48,540 --> 00:02:50,210 because they don't cost me anything. 65 65 00:02:50,210 --> 00:02:53,760 But with public IPs, every public IP I paid for, 66 66 00:02:53,760 --> 00:02:55,400 and that's money out of my pocket, 67 67 00:02:55,400 --> 00:02:56,450 and you want to minimize that 68 68 00:02:56,450 --> 00:02:58,210 to only getting the ones you need. 69 69 00:02:58,210 --> 00:03:02,870 For example, my ISP charges $5 per month for a static IP. 70 70 00:03:02,870 --> 00:03:07,080 If I need one, that's $5, if I need 10, that's $50, 71 71 00:03:07,080 --> 00:03:09,330 if I need 100, that's $500, 72 72 00:03:09,330 --> 00:03:11,540 and so, that can add up pretty quickly, right? 73 73 00:03:11,540 --> 00:03:14,000 By subnetting, it allows me to just get the things 74 74 00:03:14,000 --> 00:03:16,700 that I need and the fewer IPs that I need. 75 75 00:03:16,700 --> 00:03:18,140 Now, how do we do this? 76 76 00:03:18,140 --> 00:03:21,010 Well, that's what we use a subnet mask for. 77 77 00:03:21,010 --> 00:03:22,720 We talked about class A, class B, 78 78 00:03:22,720 --> 00:03:24,800 and class C default subnet masks 79 79 00:03:24,800 --> 00:03:26,320 back when we talked about it before, 80 80 00:03:26,320 --> 00:03:28,310 and you'll see those on the screen in red. 81 81 00:03:28,310 --> 00:03:32,370 They are the slash eight, slash 16, and slash 24. 82 82 00:03:32,370 --> 00:03:34,120 Now, when you convert those to binary, 83 83 00:03:34,120 --> 00:03:36,410 you can see that they're filled by all the octets 84 84 00:03:36,410 --> 00:03:40,830 with either all ones or all zeros, making them classful. 85 85 00:03:40,830 --> 00:03:42,530 Now, the ones on the bottom of the screen, 86 86 00:03:42,530 --> 00:03:46,460 from 25 down to slash 30, are going to be smaller subnets. 87 87 00:03:46,460 --> 00:03:48,250 These are called classless subnets 88 88 00:03:48,250 --> 00:03:49,740 because they're going to use different things 89 89 00:03:49,740 --> 00:03:53,510 besides all ones or all zeros in that octet. 90 90 00:03:53,510 --> 00:03:55,490 Now, as you can see, we're borrowing bits 91 91 00:03:55,490 --> 00:03:58,040 from the host portion and making all those zeros 92 92 00:03:58,040 --> 00:04:00,230 and taking some of those by adding them over 93 93 00:04:00,230 --> 00:04:02,310 into the network portion, as well. 94 94 00:04:02,310 --> 00:04:05,030 And so, this borrowing is what we're doing with subnetting. 95 95 00:04:05,030 --> 00:04:06,520 If you don't get it yet, don't worry, 96 96 00:04:06,520 --> 00:04:07,800 we are going to dig deeper into it 97 97 00:04:07,800 --> 00:04:09,760 as we continue through this lesson. 98 98 00:04:09,760 --> 00:04:11,430 Now, let's take a look at some formulas 99 99 00:04:11,430 --> 00:04:13,490 that we're going to need to understand for calculating 100 100 00:04:13,490 --> 00:04:16,930 the number of subnets and the number of assignable hosts. 101 101 00:04:16,930 --> 00:04:19,110 If I'm calculating the number of subnets, 102 102 00:04:19,110 --> 00:04:21,340 the formula is two to the s power, 103 103 00:04:21,340 --> 00:04:24,560 where s is the number of borrowed bits from the host. 104 104 00:04:24,560 --> 00:04:26,710 So, when I start taking those zeros from the host 105 105 00:04:26,710 --> 00:04:28,346 and turning them to ones to make them 106 106 00:04:28,346 --> 00:04:30,610 be part of the network portion, 107 107 00:04:30,610 --> 00:04:33,450 that is the s that we're talking about here. 108 108 00:04:33,450 --> 00:04:36,670 For instance, if I'm using a slash 25 as my network, 109 109 00:04:36,670 --> 00:04:38,840 I'm borrowing one bit from the host space 110 110 00:04:38,840 --> 00:04:41,070 and putting it into the network space. 111 111 00:04:41,070 --> 00:04:43,240 Now, this would make it two to the one 112 112 00:04:43,240 --> 00:04:45,560 and that would give me two, and so that tells me 113 113 00:04:45,560 --> 00:04:48,980 I can have two subnets by using a slash 25. 114 114 00:04:48,980 --> 00:04:49,890 Now, if I'm looking at the number 115 115 00:04:49,890 --> 00:04:51,580 of assignable IP addresses, 116 116 00:04:51,580 --> 00:04:55,180 this is going to be two to the number of host bits minus two. 117 117 00:04:55,180 --> 00:04:57,300 So, in my case of a slash 25 network, 118 118 00:04:57,300 --> 00:05:00,150 there are 32 total bits in the address. 119 119 00:05:00,150 --> 00:05:03,500 I have 25 of those being assigned for the network portion, 120 120 00:05:03,500 --> 00:05:05,790 leaving me with seven host bits. 121 121 00:05:05,790 --> 00:05:09,500 So, if I take two to the seventh and then I minus two, 122 122 00:05:09,500 --> 00:05:14,500 I get 128 minus two, or 126 available. 123 123 00:05:14,550 --> 00:05:17,220 Now, why do I have to have that minus two there? 124 124 00:05:17,220 --> 00:05:19,240 This is a concept that a lot of students miss, 125 125 00:05:19,240 --> 00:05:21,330 so, really pay attention to it. 126 126 00:05:21,330 --> 00:05:24,790 Every network out there has to have two IP addresses 127 127 00:05:24,790 --> 00:05:25,860 to be a network. 128 128 00:05:25,860 --> 00:05:27,780 The first is known as the network ID, 129 129 00:05:27,780 --> 00:05:31,020 and it's the first IP in the range for that network. 130 130 00:05:31,020 --> 00:05:33,090 And the other one is a broadcast ID, 131 131 00:05:33,090 --> 00:05:35,670 which is the last IP in the network. 132 132 00:05:35,670 --> 00:05:37,520 So, no matter what network you choose, 133 133 00:05:37,520 --> 00:05:40,850 you always have to sacrifice the first one and the last one. 134 134 00:05:40,850 --> 00:05:43,680 The first one is your network ID or network name, 135 135 00:05:43,680 --> 00:05:46,110 and your last one is your broadcast. 136 136 00:05:46,110 --> 00:05:49,250 Now, anytime you calculate this number of usable IPs, 137 137 00:05:49,250 --> 00:05:52,050 it's always going to be the number minus two, 138 138 00:05:52,050 --> 00:05:54,310 because you have to take away the network name 139 139 00:05:54,310 --> 00:05:56,090 and the broadcast name. 140 140 00:05:56,090 --> 00:05:59,510 Let's take a look at classful versus subnetted networks. 141 141 00:05:59,510 --> 00:06:01,060 A classful network, if you remember, 142 142 00:06:01,060 --> 00:06:05,180 are the ones like slash eight, slash 16, slash 24. 143 143 00:06:05,180 --> 00:06:10,180 And if I use the example of 192.168.1.0/24, 144 144 00:06:10,200 --> 00:06:12,980 this is a classical class C network, 145 145 00:06:12,980 --> 00:06:16,300 there is one network here because it is two to the zero, 146 146 00:06:16,300 --> 00:06:20,810 because I borrowed zero bits, because slash 24 is classful. 147 147 00:06:20,810 --> 00:06:23,200 This means that there is two to the eight 148 148 00:06:23,200 --> 00:06:27,530 minus two assignable IPs, which gives me 256 IPs, 149 149 00:06:27,530 --> 00:06:30,620 minus two, one for the broadcast, one for the network, 150 150 00:06:30,620 --> 00:06:33,980 and that gives me 254 usable IPs. 151 151 00:06:33,980 --> 00:06:35,470 As you can see here on the screen, 152 152 00:06:35,470 --> 00:06:38,450 we have all the network bits, all 24 of them, 153 153 00:06:38,450 --> 00:06:40,920 and therefore, it is classful. 154 154 00:06:40,920 --> 00:06:42,880 Now, we have eight bits of the host, 155 155 00:06:42,880 --> 00:06:45,380 and let's say that I wanted to make a smaller subnet. 156 156 00:06:45,380 --> 00:06:46,470 How can I do that? 157 157 00:06:46,470 --> 00:06:48,670 Well, I could borrow two bits from the host 158 158 00:06:48,670 --> 00:06:49,970 and create a subnet, 159 159 00:06:49,970 --> 00:06:52,570 this would make it a slash 26, for instance. 160 160 00:06:52,570 --> 00:06:54,707 So, if I borrow those two host bits, 161 161 00:06:54,707 --> 00:06:58,290 now it becomes two to the s or two to the second power, 162 162 00:06:58,290 --> 00:07:00,349 which gives me four networks, 163 163 00:07:00,349 --> 00:07:01,930 four subnets that I've created. 164 164 00:07:01,930 --> 00:07:03,730 So, if I consider those four networks, 165 165 00:07:03,730 --> 00:07:05,790 how many IPs can I have in each one 166 166 00:07:05,790 --> 00:07:07,710 of those four networks I just created? 167 167 00:07:07,710 --> 00:07:09,590 Well, in the original slash 24, 168 168 00:07:09,590 --> 00:07:13,490 I had 256 hosts total or IPs total. 169 169 00:07:13,490 --> 00:07:15,430 Now, I borrowed those two host bits 170 170 00:07:15,430 --> 00:07:16,640 and gave them to the network. 171 171 00:07:16,640 --> 00:07:19,510 And so, now I have two to the six power, 172 172 00:07:19,510 --> 00:07:22,510 that's going to give me 64 IPs per subnet. 173 173 00:07:22,510 --> 00:07:27,510 So, if I have 64 plus 64 plus 64 plus 64 that equals two 56. 174 174 00:07:28,400 --> 00:07:31,430 So, I took that one big network, that slash 24, 175 175 00:07:31,430 --> 00:07:33,700 and I turned it into four smaller networks, 176 176 00:07:33,700 --> 00:07:35,500 those slash 26s. 177 177 00:07:35,500 --> 00:07:39,320 But remember, each of those, I have to give away two IPs, 178 178 00:07:39,320 --> 00:07:40,920 the first one and the last one, 179 179 00:07:40,920 --> 00:07:42,310 the first one is the network name, 180 180 00:07:42,310 --> 00:07:44,010 the last one is the broadcast. 181 181 00:07:44,010 --> 00:07:48,330 So, I only have 62 usable IPs that I can assign 182 182 00:07:48,330 --> 00:07:51,720 to a server or a laptop or a phone or something like that. 183 183 00:07:51,720 --> 00:07:54,170 And so, for each of these four networks I've created, 184 184 00:07:54,170 --> 00:07:58,490 I now have 62 usable IPs and I have four of those 185 185 00:07:58,490 --> 00:08:01,830 that were made up out of that original slash 24 network. 186 186 00:08:01,830 --> 00:08:03,620 Now, if you look at the blue at the bottom of this, 187 187 00:08:03,620 --> 00:08:06,770 you can see my original 256 possible IPs, 188 188 00:08:06,770 --> 00:08:09,450 which does include my broadcast and my network. 189 189 00:08:09,450 --> 00:08:12,440 If I submit that down, I'm going to have four subnets 190 190 00:08:12,440 --> 00:08:14,270 or two to the second power. 191 191 00:08:14,270 --> 00:08:16,930 Now, I'm going to have 62 possible IPs, 192 192 00:08:16,930 --> 00:08:19,830 because each one was two to the sixth minus two, 193 193 00:08:19,830 --> 00:08:21,480 because I take away the first and the last 194 194 00:08:21,480 --> 00:08:23,490 for the network name and the broadcast. 195 195 00:08:23,490 --> 00:08:26,110 In our case, this is going to give us the four subnets 196 196 00:08:26,110 --> 00:08:31,110 as their name of 192.168.1.0, 192.168.1.64, 197 197 00:08:33,539 --> 00:08:38,539 192.168.1.128, and 192.168.1.192, 198 198 00:08:41,290 --> 00:08:43,200 those are my four network names. 199 199 00:08:43,200 --> 00:08:44,520 Now, for the broadcast, 200 200 00:08:44,520 --> 00:08:46,690 it's going to be the last IP of each subnet, 201 201 00:08:46,690 --> 00:08:49,920 so, that's going to be the ones ending in dot 63, 202 202 00:08:49,920 --> 00:08:54,550 dot 27, dot 191 and dot 255. 203 203 00:08:54,550 --> 00:08:56,500 So, hopefully now you're starting to see how these subnets 204 204 00:08:56,500 --> 00:08:58,040 start to work together. 205 205 00:08:58,040 --> 00:08:59,940 If we wanted to calculate those IPs, 206 206 00:08:59,940 --> 00:09:02,960 we're going to do that based on those subnet masks. 207 207 00:09:02,960 --> 00:09:04,840 So, when we take those 32 bits, 208 208 00:09:04,840 --> 00:09:06,450 which is the total number of bits 209 209 00:09:06,450 --> 00:09:09,640 minus whatever our slash or CIDR notation is, 210 210 00:09:09,640 --> 00:09:11,290 in this case 26, 211 211 00:09:11,290 --> 00:09:13,960 we get that there are six host bits remaining, 212 212 00:09:13,960 --> 00:09:15,820 that would be two to the sixth minus two, 213 213 00:09:15,820 --> 00:09:20,820 which is 64 minus two or 62 usable, assignable IPs. 214 214 00:09:21,380 --> 00:09:22,700 Now, I'm hoping that all of this 215 215 00:09:22,700 --> 00:09:24,710 is starting to come together as we've gone through it. 216 216 00:09:24,710 --> 00:09:27,510 Subnetting is a very complex topic 217 217 00:09:27,510 --> 00:09:29,300 that a lot of students struggle with. 218 218 00:09:29,300 --> 00:09:30,750 I want you to keep practicing it 219 219 00:09:30,750 --> 00:09:32,370 and keep working through the problems, 220 220 00:09:32,370 --> 00:09:34,530 because I guarantee on test day, you're going 221 221 00:09:34,530 --> 00:09:36,910 to get some questions on subnetting. 222 222 00:09:36,910 --> 00:09:38,680 Now, some of these questions might ask you 223 223 00:09:38,680 --> 00:09:40,170 just to do the problem, 224 224 00:09:40,170 --> 00:09:42,780 but most of them are going to be more about troubleshooting. 225 225 00:09:42,780 --> 00:09:44,630 There's going to be some computers that aren't talking 226 226 00:09:44,630 --> 00:09:47,210 to another, and when you start looking at their IP addresses 227 227 00:09:47,210 --> 00:09:49,220 and their subnet masks, you're going to notice 228 228 00:09:49,220 --> 00:09:50,550 they're not on the same subnet, 229 229 00:09:50,550 --> 00:09:51,990 and if there's no router there, 230 230 00:09:51,990 --> 00:09:53,990 data won't go from one subnet to another, 231 231 00:09:53,990 --> 00:09:56,110 and so, this is an important concept. 232 232 00:09:56,110 --> 00:09:59,030 Now, let's talk about how we list out these subnets. 233 233 00:09:59,030 --> 00:10:04,030 Let's keep going with our example of the 192.168.1.0/26 234 234 00:10:04,680 --> 00:10:08,890 and those four subnets we created of 62 usable IPs each. 235 235 00:10:08,890 --> 00:10:11,520 Now, where does each network begin and end? 236 236 00:10:11,520 --> 00:10:13,430 We kind of already went over this by saying 237 237 00:10:13,430 --> 00:10:18,430 it was the dot zero, the dot 64, the dot 128, and the dot 192 238 238 00:10:18,610 --> 00:10:20,370 as our beginning of each network. 239 239 00:10:20,370 --> 00:10:22,240 Now, where did I come up with those? 240 240 00:10:22,240 --> 00:10:26,157 Well, I started with the original IP, of 192.168.1.0. 241 241 00:10:27,820 --> 00:10:29,740 Now, zero is my first IP 242 242 00:10:29,740 --> 00:10:32,230 and it is going to be the name of that network. 243 243 00:10:32,230 --> 00:10:35,200 Each subnet we said was 64 IPs in length, 244 244 00:10:35,200 --> 00:10:39,100 so, if I take zero plus 64, I get dot 64, 245 245 00:10:39,100 --> 00:10:41,830 if I add another 64, I get dot 128, 246 246 00:10:41,830 --> 00:10:44,780 if I had another 64, I get dot 192. 247 247 00:10:44,780 --> 00:10:47,660 Now, how do I find the last IP for each of these? 248 248 00:10:47,660 --> 00:10:49,620 Well, that's going to be my broadcast address. 249 249 00:10:49,620 --> 00:10:53,020 All I need to do is add dot 63 to each of these numbers, 250 250 00:10:53,020 --> 00:10:56,070 because there's 64 IPs, the first one is used for the name, 251 251 00:10:56,070 --> 00:11:00,310 the last one, the plus 63 gets me to that broadcast ID, 252 252 00:11:00,310 --> 00:11:05,310 and so, I get 63, 127, 191 and 255, 253 253 00:11:05,720 --> 00:11:08,300 that's going to be the beginning and ending of each subnet. 254 254 00:11:08,300 --> 00:11:13,300 It goes from zero to 63, 64 to 127, 128 to 191, 255 255 00:11:14,080 --> 00:11:17,920 and 192 to 255, and everything in between those, 256 256 00:11:17,920 --> 00:11:20,470 for instance, I might have something like 32 257 257 00:11:20,470 --> 00:11:22,620 is going to be a usable IP. 258 258 00:11:22,620 --> 00:11:24,370 Those are things we're going to assign to clients 259 259 00:11:24,370 --> 00:11:26,070 and servers and tablets and laptops 260 260 00:11:26,070 --> 00:11:28,410 and desktops and all those things. 261 261 00:11:28,410 --> 00:11:29,989 Now, when we do all of these, 262 262 00:11:29,989 --> 00:11:31,730 we're going to write them out 263 263 00:11:31,730 --> 00:11:33,970 with what's known as the slash notation. 264 264 00:11:33,970 --> 00:11:38,970 So, we'd write 192.168.1.0/192.168.1.127, 265 265 00:11:43,070 --> 00:11:44,360 that's pretty long to write that out 266 266 00:11:44,360 --> 00:11:46,750 for each of the four subnets, so instead, 267 267 00:11:46,750 --> 00:11:50,230 we have this shorthand notation called a CIDR notation, 268 268 00:11:50,230 --> 00:11:53,240 and this stands for the Classless Inter-Domain Routing. 269 269 00:11:53,240 --> 00:11:56,150 Instead of advertising multiple individual routes, 270 270 00:11:56,150 --> 00:11:59,020 we summarize all of those as a single route. 271 271 00:11:59,020 --> 00:12:01,720 We do this through CIDR notation, and in this case, 272 272 00:12:01,720 --> 00:12:03,760 we had the slash 26. 273 273 00:12:03,760 --> 00:12:06,960 So, I can consolidate all those underneath them 274 274 00:12:06,960 --> 00:12:09,410 by doing a summary of the continuous networks 275 275 00:12:09,410 --> 00:12:11,030 using route aggregation. 276 276 00:12:11,030 --> 00:12:13,640 When I do this, and I look at the slash 26, 277 277 00:12:13,640 --> 00:12:16,680 I noticed that the first 26 bits are all equivalent, 278 278 00:12:16,680 --> 00:12:18,930 they're all the same for the subnet mask. 279 279 00:12:18,930 --> 00:12:20,660 So, by consolidating those, 280 280 00:12:20,660 --> 00:12:22,550 this allows me to put all those networks together 281 281 00:12:22,550 --> 00:12:26,380 in one slash notation and simply write them down this way. 282 282 00:12:26,380 --> 00:12:30,540 Next, we have a Variable-Length Subnet Mask or VLSM, 283 283 00:12:30,540 --> 00:12:33,660 this allows subnets of various sizes to be used. 284 284 00:12:33,660 --> 00:12:36,050 Everything I've done so far has been equal. 285 285 00:12:36,050 --> 00:12:39,610 I took a big chunk of 256 IPs and broke them up 286 286 00:12:39,610 --> 00:12:43,940 into four equal 64-bit chunks, but we don't have to do that, 287 287 00:12:43,940 --> 00:12:46,460 I could actually break it up and have a 16 chunk 288 288 00:12:46,460 --> 00:12:50,170 and a 32 chunk and a 64 chunk, and that way, I can do that 289 289 00:12:50,170 --> 00:12:52,420 to what's right for my network. 290 290 00:12:52,420 --> 00:12:54,510 Now, this is going to require a routing protocol 291 291 00:12:54,510 --> 00:12:57,720 that supports this, but all modern routing protocols do, 292 292 00:12:57,720 --> 00:13:01,533 including things like RIP, OSPF, IS-IS, EIGRP, and BGP. 293 293 00:13:03,960 --> 00:13:06,410 Now, we haven't talked much about routing protocols yet, 294 294 00:13:06,410 --> 00:13:08,420 but we will in the next section of this course 295 295 00:13:08,420 --> 00:13:10,130 when we talk about routing. 296 296 00:13:10,130 --> 00:13:12,470 Basically, Variable-Length Subnet Masking 297 297 00:13:12,470 --> 00:13:14,580 is a subnetting of subnets. 298 298 00:13:14,580 --> 00:13:18,370 Without VLSM, all subnets would have to be the same size 299 299 00:13:18,370 --> 00:13:19,940 as you see here on the screen, 300 300 00:13:19,940 --> 00:13:21,810 but by using VLSM, 301 301 00:13:21,810 --> 00:13:24,390 I can break them up into whatever chunks I want. 302 302 00:13:24,390 --> 00:13:26,640 And I'm going to give you the key to subnetting here. 303 303 00:13:26,640 --> 00:13:27,880 When you look at this chart, 304 304 00:13:27,880 --> 00:13:30,120 this is your subnetting exam tip. 305 305 00:13:30,120 --> 00:13:31,970 Write this down in your notes. 306 306 00:13:31,970 --> 00:13:33,550 Now, the one I want you to memorize here, 307 307 00:13:33,550 --> 00:13:35,620 is this small chart off to the right. 308 308 00:13:35,620 --> 00:13:37,440 You can do all the math on test day 309 309 00:13:37,440 --> 00:13:39,880 and spend a lot of time trying to do two to the s, 310 310 00:13:39,880 --> 00:13:41,710 and two to h minus two, 311 311 00:13:41,710 --> 00:13:44,740 or you can memorize this small chart on the right. 312 312 00:13:44,740 --> 00:13:47,490 If you notice, it's going to start with slash 24, 313 313 00:13:47,490 --> 00:13:50,610 and you have that as your subnet as a classful class C, 314 314 00:13:50,610 --> 00:13:52,790 you have 256 IPs, you know this, 315 315 00:13:52,790 --> 00:13:54,760 because we've talked about this a lot, right? 316 316 00:13:54,760 --> 00:13:58,500 Now, if I add one to the CIDR notation, it becomes slash 25, 317 317 00:13:58,500 --> 00:14:00,750 I increase the number of networks, 318 318 00:14:00,750 --> 00:14:03,190 by adding one, I borrowed a bit, 319 319 00:14:03,190 --> 00:14:05,680 that's going to give me two subnets, 320 320 00:14:05,680 --> 00:14:08,320 and that's going to take my 256 IPs 321 321 00:14:08,320 --> 00:14:11,300 and divide it by two, giving me 128. 322 322 00:14:11,300 --> 00:14:14,060 Notice, I'm always going to have 256 IPs 323 323 00:14:14,060 --> 00:14:18,530 as I go from slash 24 downward all the way down to slash 30. 324 324 00:14:18,530 --> 00:14:21,330 So, every time my CIDR notation goes up by one, 325 325 00:14:21,330 --> 00:14:23,460 my number of networks goes up by one 326 326 00:14:23,460 --> 00:14:26,400 and my number of IPs gets divided by two. 327 327 00:14:26,400 --> 00:14:28,370 If you can remember this chart on the right, 328 328 00:14:28,370 --> 00:14:29,960 this is going to help you on test day, 329 329 00:14:29,960 --> 00:14:32,170 get through a lot of the questions on subnetting 330 330 00:14:32,170 --> 00:14:33,810 and it's really nice and easy. 331 331 00:14:33,810 --> 00:14:35,750 Now, the good thing is for Network+, 332 332 00:14:35,750 --> 00:14:38,190 they're not going to give you a lot of difficult subnets. 333 333 00:14:38,190 --> 00:14:39,990 Almost all of the ones you're going to get 334 334 00:14:39,990 --> 00:14:42,660 are going to be slashed 24 or higher, 335 335 00:14:42,660 --> 00:14:45,460 the most are going to give you something like a slash 23 maybe, 336 336 00:14:45,460 --> 00:14:47,820 or a splash 22, but that's rare. 337 337 00:14:47,820 --> 00:14:52,070 Generally it's going to be slashed 25, 26, 27, 28, 29, and 30, 338 338 00:14:52,070 --> 00:14:53,550 that is the most common. 339 339 00:14:53,550 --> 00:14:56,210 So, if they ask you something like, you have a slash 28, 340 340 00:14:56,210 --> 00:14:57,740 how many subnets are created? 341 341 00:14:57,740 --> 00:14:59,467 It's pretty easy, you look at this chart and go, 342 342 00:14:59,467 --> 00:15:01,167 "Oh, the answer is 16." 343 343 00:15:01,167 --> 00:15:04,530 And if I go from 24 up to 28, I keep doubling it 344 344 00:15:04,530 --> 00:15:06,290 and I double it to get to 25, 345 345 00:15:06,290 --> 00:15:07,850 I double it again to get to 26, 346 346 00:15:07,850 --> 00:15:09,410 double it again to get to 27, 347 347 00:15:09,410 --> 00:15:11,860 double it again to get to 16, right? 348 348 00:15:11,860 --> 00:15:14,970 And that is where I get to slash 28 is 16, 349 349 00:15:14,970 --> 00:15:18,620 because I went two, four, eight, 16, and there we go. 350 350 00:15:18,620 --> 00:15:20,740 Now, you might ask, how many IPs you'd have in each subnet 351 351 00:15:20,740 --> 00:15:22,310 if you had a slash 30? 352 352 00:15:22,310 --> 00:15:23,930 Well, for a slash 30, 353 353 00:15:23,930 --> 00:15:26,010 that's what we use for point to point connections, 354 354 00:15:26,010 --> 00:15:27,670 and there's going to be four IPs, 355 355 00:15:27,670 --> 00:15:29,200 but only two of those are usable, 356 356 00:15:29,200 --> 00:15:31,850 because again, our first one is always the name 357 357 00:15:31,850 --> 00:15:34,460 of the network and the last one is always the broadcast. 358 358 00:15:34,460 --> 00:15:37,537 So, you have four IPs, two of which are usable. 359 359 00:15:37,537 --> 00:15:39,490 And if you remember this chart, you're going to be able 360 360 00:15:39,490 --> 00:15:42,120 to do great on subnetting on the exam. 361 361 00:15:42,120 --> 00:15:43,410 Now, before the test, 362 362 00:15:43,410 --> 00:15:46,320 I want you to take lots of practice subnetting problems, 363 363 00:15:46,320 --> 00:15:48,410 especially in your class C range. 364 364 00:15:48,410 --> 00:15:50,830 Everything from slash 24 to slash 30 365 365 00:15:50,830 --> 00:15:53,520 is really fair game for the Network+ exam. 366 366 00:15:53,520 --> 00:15:55,970 Memorizing these slash notations and the CIDR 367 367 00:15:55,970 --> 00:15:58,640 is going to help you answer any question they throw at you. 368 368 00:15:58,640 --> 00:16:00,500 But as we go through this section, 369 369 00:16:00,500 --> 00:16:01,960 I'm going to show you a shortcut 370 370 00:16:01,960 --> 00:16:04,080 that you can do just using your hands, as well, 371 371 00:16:04,080 --> 00:16:05,480 and you're going to really like this, 372 372 00:16:05,480 --> 00:16:06,670 but having this chart memorized 373 373 00:16:06,670 --> 00:16:08,360 is going to help with that, as well. 374 374 00:16:08,360 --> 00:16:11,120 Now, for the exam, something you may see is they might 375 375 00:16:11,120 --> 00:16:14,050 have you do a variable-length subnetting problem. 376 376 00:16:14,050 --> 00:16:16,300 Now, if they do a VLSM problem for you, 377 377 00:16:16,300 --> 00:16:20,340 it might be they gave you an IP address with 256 IPs 378 378 00:16:20,340 --> 00:16:22,100 because they're going to be a slash 24 379 379 00:16:22,100 --> 00:16:23,077 and then, they might say, 380 380 00:16:23,077 --> 00:16:26,647 "Hey, you have an it department that needs X number of IPs, 381 381 00:16:26,647 --> 00:16:29,387 you have HR department that needs X amount of IPs, 382 382 00:16:29,387 --> 00:16:32,580 you have a sales department that needs Y amount of IPs." 383 383 00:16:32,580 --> 00:16:34,980 And you'll have to go through and do the slash notations 384 384 00:16:34,980 --> 00:16:37,900 for each one based on the number of IPs they give you. 385 385 00:16:37,900 --> 00:16:40,230 That is a totally fair question on the exam 386 386 00:16:40,230 --> 00:16:42,420 and how you'd put this subnetting stuff to work 387 387 00:16:42,420 --> 00:16:43,423 in the real world.