1 1 00:00:00,480 --> 00:00:02,550 Hands-on with DHCP. 2 2 00:00:02,550 --> 00:00:05,240 In this lesson, I'm going to show you DHCP 3 3 00:00:05,240 --> 00:00:06,980 and how it works behind the scenes 4 4 00:00:06,980 --> 00:00:09,700 by looking at the packets as they go across our network. 5 5 00:00:09,700 --> 00:00:11,310 Now, when we look at DHCP here, 6 6 00:00:11,310 --> 00:00:13,320 I'm going to do it in a very simplistic manner 7 7 00:00:13,320 --> 00:00:15,320 and I'm going to use a small office home office 8 8 00:00:15,320 --> 00:00:17,400 wireless router or wireless gateway. 9 9 00:00:17,400 --> 00:00:20,100 You can see that here on the left side of the screen. 10 10 00:00:20,100 --> 00:00:21,790 All right, so let's go ahead and get started here. 11 11 00:00:21,790 --> 00:00:23,490 We have a wireless router at the top 12 12 00:00:23,490 --> 00:00:26,590 and I've got two clients, PC0 and PC1. 13 13 00:00:26,590 --> 00:00:29,170 Right now, PC0 is statically assigned 14 14 00:00:29,170 --> 00:00:31,600 and PC1 is dynamically assigned. 15 15 00:00:31,600 --> 00:00:33,050 You can see that if I click on them. 16 16 00:00:33,050 --> 00:00:34,270 Here's PC1. 17 17 00:00:34,270 --> 00:00:36,020 You can see it is set up for DHCP 18 18 00:00:36,020 --> 00:00:40,010 and it's pulled an address of 192.168.0.1 19 19 00:00:40,010 --> 00:00:41,310 as its default gateway 20 20 00:00:41,310 --> 00:00:43,170 and if I look at its actual interface card, 21 21 00:00:43,170 --> 00:00:48,170 you could see its IP address over here at 192.168.0.101. 22 22 00:00:48,650 --> 00:00:50,230 Now if I look at PC0, 23 23 00:00:50,230 --> 00:00:52,040 you're going to see on PC0 that I have 24 24 00:00:52,040 --> 00:00:53,950 a statically assigned default gateway, 25 25 00:00:53,950 --> 00:00:56,080 and that gateway doesn't match the DHCP one, 26 26 00:00:56,080 --> 00:00:57,750 which is why I'm having some communication problems 27 27 00:00:57,750 --> 00:00:58,583 with this one. 28 28 00:00:58,583 --> 00:01:01,020 So I decided I'm just going to make my life easy 29 29 00:01:01,020 --> 00:01:02,610 and set up DHCP. 30 30 00:01:02,610 --> 00:01:04,720 So I'm going to click on DHCP 31 31 00:01:04,720 --> 00:01:07,290 and it will automatically switch me to DHCP. 32 32 00:01:07,290 --> 00:01:08,940 Now, once I click on DHCP, 33 33 00:01:08,940 --> 00:01:11,350 I'm going to minimize this window and that way we can see it 34 34 00:01:11,350 --> 00:01:13,650 as it steps through the DORA process. 35 35 00:01:13,650 --> 00:01:15,160 So the first packet that's going to go out 36 36 00:01:15,160 --> 00:01:18,450 is going to be a broadcast message for the discover process. 37 37 00:01:18,450 --> 00:01:20,060 Notice here, the destination. 38 38 00:01:20,060 --> 00:01:24,070 It is 255.255.255.255. 39 39 00:01:24,070 --> 00:01:26,810 And this tells us that that is the broadcast 40 40 00:01:26,810 --> 00:01:27,950 for layer three. 41 41 00:01:27,950 --> 00:01:28,950 And so we're going to send this out 42 42 00:01:28,950 --> 00:01:30,710 and everybody connected over layer three 43 43 00:01:30,710 --> 00:01:32,300 is going to get this message. 44 44 00:01:32,300 --> 00:01:33,830 In this case, the only have the person 45 45 00:01:33,830 --> 00:01:36,880 that's connected over layer three is wireless router zero. 46 46 00:01:36,880 --> 00:01:39,450 So when I do this and I hit the next button, 47 47 00:01:39,450 --> 00:01:40,950 we're going to send that packet up. 48 48 00:01:40,950 --> 00:01:42,760 It gets to wireless router zero. 49 49 00:01:42,760 --> 00:01:45,570 When it gets there, it accepts that and it processes it. 50 50 00:01:45,570 --> 00:01:47,980 It says, oh, somebody on this network 51 51 00:01:47,980 --> 00:01:49,610 wants to get an IP address. 52 52 00:01:49,610 --> 00:01:51,300 So this was the discovery phase. 53 53 00:01:51,300 --> 00:01:53,950 Since this wireless router has a DHCP server, 54 54 00:01:53,950 --> 00:01:55,210 it's going to say, great. 55 55 00:01:55,210 --> 00:01:58,000 I can give you an IP address and I'm going to offer you one. 56 56 00:01:58,000 --> 00:01:59,800 So it's going to send a packet back. 57 57 00:01:59,800 --> 00:02:01,140 And when it sends that packet, 58 58 00:02:01,140 --> 00:02:03,150 we are going to send that out as a broadcast. 59 59 00:02:03,150 --> 00:02:06,210 So it goes to both PC0 and PC1. 60 60 00:02:06,210 --> 00:02:09,430 In this case, PC0 accepted it and PC1 didn't. 61 61 00:02:09,430 --> 00:02:11,000 Why didn't PC1 accept it? 62 62 00:02:11,000 --> 00:02:12,380 Because it didn't ask for it. 63 63 00:02:12,380 --> 00:02:13,780 It didn't do the discover process. 64 64 00:02:13,780 --> 00:02:14,927 So it says, this isn't meant for me. 65 65 00:02:14,927 --> 00:02:17,100 And it simply drops that packet. 66 66 00:02:17,100 --> 00:02:18,610 Now, on the other hand, PC0 says 67 67 00:02:18,610 --> 00:02:20,010 I was looking for a message. 68 68 00:02:20,010 --> 00:02:21,970 And so this message came as a broadcast. 69 69 00:02:21,970 --> 00:02:24,720 You could see the destination there is listed as broadcast. 70 70 00:02:24,720 --> 00:02:26,760 Now because this was sent as a broadcast message, 71 71 00:02:26,760 --> 00:02:29,090 it's coming over layer three at the broadcast. 72 72 00:02:29,090 --> 00:02:31,410 So again, you could see the destination IP address 73 73 00:02:31,410 --> 00:02:34,680 of 255.255.255.255. 74 74 00:02:34,680 --> 00:02:37,390 So everyone on this network got a copy of that, 75 75 00:02:37,390 --> 00:02:39,440 but only PC0 wanted it, 76 76 00:02:39,440 --> 00:02:41,640 so it's the only one that's going to process that message 77 77 00:02:41,640 --> 00:02:44,420 as indicated by those blue lines going over it. 78 78 00:02:44,420 --> 00:02:46,260 So next, we're going to get that. 79 79 00:02:46,260 --> 00:02:48,710 And we've discovered where we asked for it. 80 80 00:02:48,710 --> 00:02:50,340 We received it as an offer. 81 81 00:02:50,340 --> 00:02:52,500 So now we need to request it and say, 82 82 00:02:52,500 --> 00:02:54,350 can I keep that address? 83 83 00:02:54,350 --> 00:02:55,410 Now remember, we just send that out 84 84 00:02:55,410 --> 00:02:56,520 to everybody in the network. 85 85 00:02:56,520 --> 00:02:58,060 So maybe there was two or three clients 86 86 00:02:58,060 --> 00:02:59,290 that wanted that address. 87 87 00:02:59,290 --> 00:03:00,690 So whichever one requests it 88 88 00:03:00,690 --> 00:03:02,970 is then going to get an acknowledgement from the server. 89 89 00:03:02,970 --> 00:03:06,930 So PC0 is going to go back and say, I want this IP address. 90 90 00:03:06,930 --> 00:03:09,880 And so it sends its request as part of the DORA process. 91 91 00:03:09,880 --> 00:03:11,860 That goes up to wireless router zero. 92 92 00:03:11,860 --> 00:03:13,720 When it gets it, it's going to see it. 93 93 00:03:13,720 --> 00:03:16,370 And in this case, we are sending this as again, 94 94 00:03:16,370 --> 00:03:18,750 a broadcast message because everything is being done 95 95 00:03:18,750 --> 00:03:21,310 in broadcasting when you're using DHCP. 96 96 00:03:21,310 --> 00:03:24,060 Now, once it gets up there and wireless router zero sees it, 97 97 00:03:24,060 --> 00:03:25,510 it's going to process that message and say, 98 98 00:03:25,510 --> 00:03:28,680 okay, I acknowledge that PC0 can have this. 99 99 00:03:28,680 --> 00:03:30,710 And so it sends out another message to everybody, 100 100 00:03:30,710 --> 00:03:32,460 and this goes to all the PCs. 101 101 00:03:32,460 --> 00:03:35,350 And again, just like before, this is a broadcast message. 102 102 00:03:35,350 --> 00:03:37,110 So if it wasn't intended for you, 103 103 00:03:37,110 --> 00:03:38,460 you're just going to drop it. 104 104 00:03:38,460 --> 00:03:41,290 This case, PC0 wanted it, so it accepts it 105 105 00:03:41,290 --> 00:03:42,830 and we get that green check mark. 106 106 00:03:42,830 --> 00:03:46,760 At this point, PC0 should now have an IP address. 107 107 00:03:46,760 --> 00:03:49,300 So if we go into PC0 we should now see 108 108 00:03:49,300 --> 00:03:51,360 that there is a DHCP address assigned. 109 109 00:03:51,360 --> 00:03:52,970 Now notice we have a default gateway 110 110 00:03:52,970 --> 00:03:56,780 of 192.168.0.1 that has been assigned 111 111 00:03:56,780 --> 00:03:59,490 as a dynamic DHCP gateway for us. 112 112 00:03:59,490 --> 00:04:01,700 Now, if we look at the wireless interface itself, 113 113 00:04:01,700 --> 00:04:03,310 we will see its actual IP address. 114 114 00:04:03,310 --> 00:04:06,960 In this case, 192.168.0.100 115 115 00:04:06,960 --> 00:04:10,870 with a subnet mask of 255.255.255.0, 116 116 00:04:10,870 --> 00:04:13,180 which means we are now on this network. 117 117 00:04:13,180 --> 00:04:15,560 We have a valid IP and we can start to communicate 118 118 00:04:15,560 --> 00:04:17,030 with other computers on this network 119 119 00:04:17,030 --> 00:04:20,030 or out through that default gateway over to the internet, 120 120 00:04:20,030 --> 00:04:22,220 if that wireless router is connected to a cable modem 121 121 00:04:22,220 --> 00:04:23,900 or something else like that. 122 122 00:04:23,900 --> 00:04:26,230 All right, that is the quick and dirty way 123 123 00:04:26,230 --> 00:04:28,813 of how DHCP works through the DORA process. 124 124 00:04:28,813 --> 00:04:30,520 Now, the other thing we might want to look at 125 125 00:04:30,520 --> 00:04:32,720 is inside the wireless router itself. 126 126 00:04:32,720 --> 00:04:34,050 Now I'm going to go ahead and maximize this 127 127 00:04:34,050 --> 00:04:35,370 so you can see it a little better. 128 128 00:04:35,370 --> 00:04:38,470 And over here on the front page, you see the DHCP server. 129 129 00:04:38,470 --> 00:04:39,620 It is enabled here. 130 130 00:04:39,620 --> 00:04:41,930 That's why this wireless router was entering up 131 131 00:04:41,930 --> 00:04:43,600 and handing out IP addresses, 132 132 00:04:43,600 --> 00:04:45,710 because it has a DHCP server enabled. 133 133 00:04:45,710 --> 00:04:48,010 And you can see that the router has an IP address 134 134 00:04:48,010 --> 00:04:51,180 of one 192.168.0.1. 135 135 00:04:51,180 --> 00:04:52,760 This is the default gateway 136 136 00:04:52,760 --> 00:04:55,320 and this router is acting as that default gateway. 137 137 00:04:55,320 --> 00:04:59,830 We also see it subnet mask, 255.255.255.0. 138 138 00:04:59,830 --> 00:05:02,720 That means that we have 256 total IPs, 139 139 00:05:02,720 --> 00:05:03,900 one for the broadcast, 140 140 00:05:03,900 --> 00:05:06,090 one for the network name and 254 141 141 00:05:06,090 --> 00:05:07,530 that are reserved for clients. 142 142 00:05:07,530 --> 00:05:09,940 Now, when it comes to handing out DHCP addresses, 143 143 00:05:09,940 --> 00:05:11,400 where are we going to start that? 144 144 00:05:11,400 --> 00:05:13,670 Well, here you can see the start IP address. 145 145 00:05:13,670 --> 00:05:17,430 It's set at 192.168.0.100. 146 146 00:05:17,430 --> 00:05:20,660 Now, the maximum number of users we're going to hand out is 50. 147 147 00:05:20,660 --> 00:05:22,920 That means we're only going to hand out IP addresses 148 148 00:05:22,920 --> 00:05:26,780 that start with 1920.168.0.100 149 149 00:05:26,780 --> 00:05:31,220 all the way up to 192.168.0.149, 150 150 00:05:31,220 --> 00:05:33,930 giving me 50 total clients that we can use. 151 151 00:05:33,930 --> 00:05:35,790 Now, if you have a particular device that you want 152 152 00:05:35,790 --> 00:05:37,500 to always get the same IP address, 153 153 00:05:37,500 --> 00:05:40,560 like maybe a wireless printer or a laptop or a server, 154 154 00:05:40,560 --> 00:05:43,190 you can do that using DHCP reservations. 155 155 00:05:43,190 --> 00:05:45,940 In this device, you just click DHCP reservations 156 156 00:05:45,940 --> 00:05:47,700 and here you'll see it's already been configured 157 157 00:05:47,700 --> 00:05:49,090 so that there are two devices 158 158 00:05:49,090 --> 00:05:51,010 that are getting IP addresses this way. 159 159 00:05:51,010 --> 00:05:54,813 We have one set 192.168.0.101, 160 160 00:05:54,813 --> 00:05:56,350 and we know which one that was. 161 161 00:05:56,350 --> 00:05:57,660 That was PC1. 162 162 00:05:57,660 --> 00:06:01,190 And then we have 192.168.0.100, 163 163 00:06:01,190 --> 00:06:03,600 which was PC0, the one we just turned on 164 164 00:06:03,600 --> 00:06:05,550 and went through the DORA process for. 165 165 00:06:05,550 --> 00:06:07,530 This means that anytime I turn off that thing 166 166 00:06:07,530 --> 00:06:09,870 and I put it at static and I come back to doing dynamic, 167 167 00:06:09,870 --> 00:06:11,620 I'm always going to get the same address 168 168 00:06:11,620 --> 00:06:13,440 because this is what I reserved for it. 169 169 00:06:13,440 --> 00:06:15,830 Now, if I didn't care what address I had for this device, 170 170 00:06:15,830 --> 00:06:17,600 I can go ahead and remove the mapping. 171 171 00:06:17,600 --> 00:06:18,750 In the instance of a laptop, 172 172 00:06:18,750 --> 00:06:20,820 I really don't care which IP it has, 173 173 00:06:20,820 --> 00:06:22,830 but if I had a printer or a file server 174 174 00:06:22,830 --> 00:06:24,090 or something like that, 175 175 00:06:24,090 --> 00:06:26,200 I might want to have a DHCP reservation 176 176 00:06:26,200 --> 00:06:28,010 so it doesn't change each time. 177 177 00:06:28,010 --> 00:06:29,730 If you want to add something to this list, 178 178 00:06:29,730 --> 00:06:30,750 you can just give it a name. 179 179 00:06:30,750 --> 00:06:32,450 For instance, let's say I have a printer. 180 180 00:06:32,450 --> 00:06:34,830 I'm going to say that is going to be 103 181 181 00:06:34,830 --> 00:06:37,160 and I'm going to save the Mac address for that printer, 182 182 00:06:37,160 --> 00:06:42,110 and nominally I'm just going to say it's 112233445566, 183 183 00:06:42,110 --> 00:06:43,780 and then I'll hit add, and you'll see 184 184 00:06:43,780 --> 00:06:45,600 that it is now being reserved. 185 185 00:06:45,600 --> 00:06:46,980 Down here we have printer. 186 186 00:06:46,980 --> 00:06:50,810 We have the IP address of 192.168.0.103 187 187 00:06:50,810 --> 00:06:53,160 and the Mac address that I just assigned. 188 188 00:06:53,160 --> 00:06:55,120 If I no longer want to have that reservation, 189 189 00:06:55,120 --> 00:06:57,200 I simply go over and click remove. 190 190 00:06:57,200 --> 00:06:59,250 Then, because this is a graphical user interface, 191 191 00:06:59,250 --> 00:07:02,100 always click save settings so that your settings are saved 192 192 00:07:02,100 --> 00:07:04,100 and it is applied to that router. 193 193 00:07:04,100 --> 00:07:06,870 All right, hopefully you enjoyed that little demonstration 194 194 00:07:06,870 --> 00:07:09,520 with DHCP and it helps you understand a little better 195 195 00:07:09,520 --> 00:07:12,800 how the whole DORA process works with the DHCP server 196 196 00:07:12,800 --> 00:07:15,500 and when you might want to use scopes and reservations 197 197 00:07:15,500 --> 00:07:17,133 inside your DHCP setup.