1 1 00:00:00,820 --> 00:00:02,420 Now that we're at the network layer, 2 2 00:00:02,420 --> 00:00:04,220 we're concerned with routing. 3 3 00:00:04,220 --> 00:00:07,200 Layer 3 is all about how we're going to forward traffic, 4 4 00:00:07,200 --> 00:00:10,720 which we refer to as routing using logical addresses. 5 5 00:00:10,720 --> 00:00:14,120 For example, your computer has an IP address. 6 6 00:00:14,120 --> 00:00:17,510 And that IP address is either going to be an IP version 4 7 7 00:00:17,510 --> 00:00:20,370 or an IP version 6 address, or both. 8 8 00:00:20,370 --> 00:00:23,280 Now, both of these are considered layer 3 protocols. 9 9 00:00:23,280 --> 00:00:24,540 And we're going to talk more about them 10 10 00:00:24,540 --> 00:00:26,510 as we go through this course. 11 11 00:00:26,510 --> 00:00:28,590 Now, the other thing we're going to be concerned with here 12 12 00:00:28,590 --> 00:00:30,420 is logical addressing. 13 13 00:00:30,420 --> 00:00:32,850 I mentioned that IPv4 and IPv6 14 14 00:00:32,850 --> 00:00:35,010 are two types of logical addresses, 15 15 00:00:35,010 --> 00:00:37,550 but they're not the only logical addressing schemes 16 16 00:00:37,550 --> 00:00:38,520 that are out there. 17 17 00:00:38,520 --> 00:00:41,960 They're just the most common and most popular these days. 18 18 00:00:41,960 --> 00:00:43,350 Now, we're also going to be concerned 19 19 00:00:43,350 --> 00:00:44,960 with what's known as switching. 20 20 00:00:44,960 --> 00:00:46,750 And here, when we talk about switching, 21 21 00:00:46,750 --> 00:00:49,350 we're actually talking about layer 3 switching, 22 22 00:00:49,350 --> 00:00:50,830 which is called routing. 23 23 00:00:50,830 --> 00:00:52,190 Now, I know this gets confusing 24 24 00:00:52,190 --> 00:00:54,010 because you're using the term switching 25 25 00:00:54,010 --> 00:00:56,010 to refer to the function of routing, 26 26 00:00:56,010 --> 00:00:57,840 and when we talk about switches, 27 27 00:00:57,840 --> 00:01:00,130 the devices being layer 2 devices. 28 28 00:01:00,130 --> 00:01:01,730 So, you have to keep this straight in your head. 29 29 00:01:01,730 --> 00:01:04,750 Switches, the physical device, are layer 2. 30 30 00:01:04,750 --> 00:01:07,190 Switching is another term for routing, 31 31 00:01:07,190 --> 00:01:08,790 which is how we transfer things 32 32 00:01:08,790 --> 00:01:11,590 at the network layer, layer 3. 33 33 00:01:11,590 --> 00:01:14,450 Now, as we talk about all these, another thing that comes up 34 34 00:01:14,450 --> 00:01:17,800 is how we're going to do route discovery and selection. 35 35 00:01:17,800 --> 00:01:19,980 Now, basically, that means how do I know 36 36 00:01:19,980 --> 00:01:22,180 which way I want the traffic to go? 37 37 00:01:22,180 --> 00:01:23,980 We're going to talk a little bit more about that, too, 38 38 00:01:23,980 --> 00:01:25,500 as we go through this lesson 39 39 00:01:25,500 --> 00:01:27,570 because we're going to talk about connection services 40 40 00:01:27,570 --> 00:01:31,010 and bandwidth utilization and multiplexing strategies. 41 41 00:01:31,010 --> 00:01:34,550 All of these is at the layer 3 of the network layer. 42 42 00:01:34,550 --> 00:01:36,950 So, to start diving in deeper into these concepts, 43 43 00:01:36,950 --> 00:01:39,400 let's start out with logical addresses. 44 44 00:01:39,400 --> 00:01:41,660 There are lots of different routed protocols 45 45 00:01:41,660 --> 00:01:43,440 that have been used over the years. 46 46 00:01:43,440 --> 00:01:45,140 Back in the '80s and '90s, 47 47 00:01:45,140 --> 00:01:47,670 there was AppleTalk for Apple computers. 48 48 00:01:47,670 --> 00:01:50,730 And if you used a Windows or a Novell Network computer, 49 49 00:01:50,730 --> 00:01:52,300 you might have used IPX, 50 50 00:01:52,300 --> 00:01:54,870 which was the Internetwork Packet Exchange. 51 51 00:01:54,870 --> 00:01:56,660 Now, neither of these two are important 52 52 00:01:56,660 --> 00:01:58,560 for the Network+ certification, 53 53 00:01:58,560 --> 00:02:00,080 but it is something that I want to bring up 54 54 00:02:00,080 --> 00:02:02,160 because you may hear these terms. 55 55 00:02:02,160 --> 00:02:05,050 Really, what happened was these were both killed off 56 56 00:02:05,050 --> 00:02:07,970 by Internet Protocol, which is known as IP. 57 57 00:02:07,970 --> 00:02:10,410 This became the common protocol that everyone uses 58 58 00:02:10,410 --> 00:02:12,220 on all networks and therefore, 59 59 00:02:12,220 --> 00:02:15,350 we didn't need AppleTalk and we didn't need IPX anymore. 60 60 00:02:15,350 --> 00:02:18,960 But the point is, at layer 3, it's not just IP. 61 61 00:02:18,960 --> 00:02:22,430 There are other protocols that you could use in layer 3. 62 62 00:02:22,430 --> 00:02:24,810 IP is just the most common. 63 63 00:02:24,810 --> 00:02:26,690 Now, some of those are still existing 64 64 00:02:26,690 --> 00:02:28,010 on some legacy systems, 65 65 00:02:28,010 --> 00:02:31,070 which means old systems in some corporate network. 66 66 00:02:31,070 --> 00:02:33,950 But the routing protocol of the Internet that we use 67 67 00:02:33,950 --> 00:02:35,290 and the Internet you have at home 68 68 00:02:35,290 --> 00:02:39,030 and the network you have at home is going to be known as IP. 69 69 00:02:39,030 --> 00:02:43,620 IP comes in two variants, as I said before, IPv4 and IPv6. 70 70 00:02:44,580 --> 00:02:46,270 Now, if you look on the screen here, 71 71 00:02:46,270 --> 00:02:48,510 there's an example of an IP address. 72 72 00:02:48,510 --> 00:02:49,763 It's written as 172 73 73 00:02:50,904 --> 00:02:52,317 .16 74 74 00:02:52,317 --> 00:02:53,627 .254 75 75 00:02:53,627 --> 00:02:54,550 .1. 76 76 00:02:54,550 --> 00:02:56,350 Now, we're going to look at more IP addresses 77 77 00:02:56,350 --> 00:03:00,090 in a separate lesson as we dig deeper into routing later on. 78 78 00:03:00,090 --> 00:03:03,230 For the time being, I want you to think of an IP address 79 79 00:03:03,230 --> 00:03:05,470 anytime you see a number that looks like this. 80 80 00:03:05,470 --> 00:03:08,540 There's going to be four sets of numbers, separated by dots. 81 81 00:03:08,540 --> 00:03:10,970 This is called a dotted-octet notation, 82 82 00:03:10,970 --> 00:03:14,180 and this is what an IPv4 address is going to look like. 83 83 00:03:14,180 --> 00:03:15,910 Now, how shall we actually forward 84 84 00:03:15,910 --> 00:03:18,270 or route the data across our networks? 85 85 00:03:18,270 --> 00:03:21,240 This is really the big question at layer 3. 86 86 00:03:21,240 --> 00:03:23,740 And there are three main ways for us to do this. 87 87 00:03:23,740 --> 00:03:25,210 You can use packet switching, 88 88 00:03:25,210 --> 00:03:27,520 circuit switching, or message switching. 89 89 00:03:27,520 --> 00:03:29,550 The most commonly used one in your network 90 90 00:03:29,550 --> 00:03:30,730 is going to be routing, 91 91 00:03:30,730 --> 00:03:32,830 which is also known as packet switching. 92 92 00:03:32,830 --> 00:03:35,000 This is where data is divided into packets 93 93 00:03:35,000 --> 00:03:37,120 and then each packet is forwarded on 94 94 00:03:37,120 --> 00:03:39,010 based on its IP address. 95 95 00:03:39,010 --> 00:03:41,330 Now, when I think of routing, I like to think about this 96 96 00:03:41,330 --> 00:03:44,220 as if I'm going to write a letter and send it to my mom. 97 97 00:03:44,220 --> 00:03:45,940 Let's say I put that letter in an envelope, 98 98 00:03:45,940 --> 00:03:47,450 and on the outside of the envelope, 99 99 00:03:47,450 --> 00:03:49,320 I'm going to write the address of my mom on it. 100 100 00:03:49,320 --> 00:03:51,660 I put her city and her state and the zip code. 101 101 00:03:51,660 --> 00:03:53,060 Now, I put that in the mailbox, 102 102 00:03:53,060 --> 00:03:56,280 and the mail carrier is going to take it to a central location. 103 103 00:03:56,280 --> 00:03:58,760 Here, they're going to figure out what state it goes to 104 104 00:03:58,760 --> 00:04:01,690 and then, they're going to send it to that state's post office. 105 105 00:04:01,690 --> 00:04:04,250 Once it's in that state, it's going to go down even further, 106 106 00:04:04,250 --> 00:04:06,820 and they're going to go down to the city-level post office. 107 107 00:04:06,820 --> 00:04:09,220 And then, from that city-level post office, 108 108 00:04:09,220 --> 00:04:11,270 they're going to look at where the street address is, 109 109 00:04:11,270 --> 00:04:12,340 get it to the right street, 110 110 00:04:12,340 --> 00:04:15,310 and eventually get it to her house on that street. 111 111 00:04:15,310 --> 00:04:18,500 The same kind of concept works with IP addresses. 112 112 00:04:18,500 --> 00:04:19,910 And that's the idea here. 113 113 00:04:19,910 --> 00:04:22,230 It's going to keep going and switching that packet 114 114 00:04:22,230 --> 00:04:25,480 from place to place until it gets to its final destination. 115 115 00:04:25,480 --> 00:04:28,310 In the case of my envelope, to my mom's address. 116 116 00:04:28,310 --> 00:04:30,020 Now, that's the way that packet switching 117 117 00:04:30,020 --> 00:04:31,410 is going to work for us. 118 118 00:04:31,410 --> 00:04:32,820 Every time I send a letter out, 119 119 00:04:32,820 --> 00:04:34,760 it might take a different route to get there. 120 120 00:04:34,760 --> 00:04:36,530 And I really don't care which route it takes, 121 121 00:04:36,530 --> 00:04:39,120 as long as it gets to its final destination. 122 122 00:04:39,120 --> 00:04:41,500 It's the same thing with our packets in the network. 123 123 00:04:41,500 --> 00:04:43,590 When I talk about circuit switching, though, 124 124 00:04:43,590 --> 00:04:44,680 this is where we want to have 125 125 00:04:44,680 --> 00:04:47,070 the same path each and every time. 126 126 00:04:47,070 --> 00:04:49,250 We're going to get a dedicated communications link 127 127 00:04:49,250 --> 00:04:51,290 that's established between our two devices. 128 128 00:04:51,290 --> 00:04:53,440 And if I pick up the phone to make a phone call, 129 129 00:04:53,440 --> 00:04:56,560 I'm actually going to make a virtual connection from my phone 130 130 00:04:56,560 --> 00:04:59,610 over to the other receiver's phone on the other end. 131 131 00:04:59,610 --> 00:05:01,350 So, if I pick up the phone to call you, 132 132 00:05:01,350 --> 00:05:03,160 there's going to be a temporary connection made 133 133 00:05:03,160 --> 00:05:05,030 between my phone and your phone, 134 134 00:05:05,030 --> 00:05:07,360 and all of the data that we're talking back and forth 135 135 00:05:07,360 --> 00:05:10,440 will go across the same path to get from me to you. 136 136 00:05:10,440 --> 00:05:12,150 The whole time we have this conversation going on, 137 137 00:05:12,150 --> 00:05:13,600 that's what's going to happen. 138 138 00:05:13,600 --> 00:05:16,360 That's what we call a circuit switch connection, 139 139 00:05:16,360 --> 00:05:18,920 which is different than the packet switch connection 140 140 00:05:18,920 --> 00:05:20,610 of using an envelope where we don't care 141 141 00:05:20,610 --> 00:05:21,820 where all those envelopes go 142 142 00:05:21,820 --> 00:05:24,230 as long as they got to the right place at the end. 143 143 00:05:24,230 --> 00:05:25,960 Now, when we hang up the phone 144 144 00:05:25,960 --> 00:05:27,310 and we go to make another call, 145 145 00:05:27,310 --> 00:05:29,850 it could take a different path, and that's okay. 146 146 00:05:29,850 --> 00:05:32,810 But for the entire session of us having our phone call, 147 147 00:05:32,810 --> 00:05:34,270 we want the same path, 148 148 00:05:34,270 --> 00:05:36,980 and that's what circuit switching allows us to do. 149 149 00:05:36,980 --> 00:05:38,810 Now, the third type of switching we have 150 150 00:05:38,810 --> 00:05:40,720 is known as message switching. 151 151 00:05:40,720 --> 00:05:41,910 And with message switching, 152 152 00:05:41,910 --> 00:05:44,530 this is where all the data is divided into messages, 153 153 00:05:44,530 --> 00:05:47,000 and they're similar to packet switching in this idea. 154 154 00:05:47,000 --> 00:05:49,160 But the messages can actually be stored 155 155 00:05:49,160 --> 00:05:51,600 and forwarded, more like email. 156 156 00:05:51,600 --> 00:05:53,760 So, if you go back to my mail example, 157 157 00:05:53,760 --> 00:05:55,270 maybe it gets to my mom's state, 158 158 00:05:55,270 --> 00:05:57,930 but the post office is closed because it's Sunday. 159 159 00:05:57,930 --> 00:05:59,460 So, what happens is they drop 160 160 00:05:59,460 --> 00:06:01,620 all those envelopes on the floor. 161 161 00:06:01,620 --> 00:06:04,480 Now, it's going to be held there until they open again 162 162 00:06:04,480 --> 00:06:06,400 on Monday and somebody's going to be able to pick it up, 163 163 00:06:06,400 --> 00:06:08,760 figure out where it goes, and push it along. 164 164 00:06:08,760 --> 00:06:09,740 This is what happens 165 165 00:06:09,740 --> 00:06:11,400 when you're dealing with message switching 166 166 00:06:11,400 --> 00:06:13,850 because it has a store and forward capability. 167 167 00:06:13,850 --> 00:06:15,820 If we were using just packet switching, 168 168 00:06:15,820 --> 00:06:17,950 what would end up happening is if it got to the post office 169 169 00:06:17,950 --> 00:06:19,530 and the post office was closed, 170 170 00:06:19,530 --> 00:06:21,470 it would actually just shred that envelope 171 171 00:06:21,470 --> 00:06:22,900 and nobody would ever see it. 172 172 00:06:22,900 --> 00:06:24,440 That's a bad thing if we want to make sure 173 173 00:06:24,440 --> 00:06:26,030 the data is going to get where it's going, 174 174 00:06:26,030 --> 00:06:29,800 and that's why message switching can be very useful for us. 175 175 00:06:29,800 --> 00:06:31,810 Now, almost all of our networks nowadays 176 176 00:06:31,810 --> 00:06:33,090 and the ones you utilize 177 177 00:06:33,090 --> 00:06:35,210 are going to be using packet switching, though. 178 178 00:06:35,210 --> 00:06:38,380 And the reason is we have other methods that will check 179 179 00:06:38,380 --> 00:06:40,490 if something is not getting to the distant end, 180 180 00:06:40,490 --> 00:06:42,410 and will be resent over another path 181 181 00:06:42,410 --> 00:06:44,000 until it finally gets there. 182 182 00:06:44,000 --> 00:06:44,833 So, unless you're dealing 183 183 00:06:44,833 --> 00:06:47,030 with some kind of big backend networks, 184 184 00:06:47,030 --> 00:06:49,130 you're not really going to see something like circuit switching 185 185 00:06:49,130 --> 00:06:52,120 or message switching in your normal everyday networks. 186 186 00:06:52,120 --> 00:06:54,720 Your home network, my small office network, 187 187 00:06:54,720 --> 00:06:56,070 and most of the Internet 188 188 00:06:56,070 --> 00:06:58,210 actually works using packet switching. 189 189 00:06:58,210 --> 00:06:59,940 Now, the second thing we have to talk about 190 190 00:06:59,940 --> 00:07:02,070 is route discovery and selection, 191 191 00:07:02,070 --> 00:07:03,730 how are we going to decide which path 192 192 00:07:03,730 --> 00:07:06,240 we're going to take to send that message. 193 193 00:07:06,240 --> 00:07:09,010 Well, routers are going to maintain a routing table 194 194 00:07:09,010 --> 00:07:11,090 so, they can understand how to forward a packet 195 195 00:07:11,090 --> 00:07:14,240 based on the destination IP of where it wants to get to. 196 196 00:07:14,240 --> 00:07:16,370 There are lots of different ways that it can do this, 197 197 00:07:16,370 --> 00:07:18,440 and they can do this either as a static route 198 198 00:07:18,440 --> 00:07:19,950 or dynamically-assigned route 199 199 00:07:19,950 --> 00:07:24,950 using a routing protocol like RIP, OSPF, EIGRP, 200 200 00:07:24,960 --> 00:07:26,170 and many others. 201 201 00:07:26,170 --> 00:07:27,850 Now, we're going to talk about many of those 202 202 00:07:27,850 --> 00:07:30,100 later on in this course, so we're not going to talk 203 203 00:07:30,100 --> 00:07:32,460 specifically about how it works right now. 204 204 00:07:32,460 --> 00:07:34,140 We're just going to put that to the side. 205 205 00:07:34,140 --> 00:07:36,500 But I want you to remember that routing protocols 206 206 00:07:36,500 --> 00:07:39,960 help us decide how data is going to flow across the network 207 207 00:07:39,960 --> 00:07:42,980 and how the routers are going to communicate that information. 208 208 00:07:42,980 --> 00:07:45,380 For now, let's just use the example on the screen 209 209 00:07:45,380 --> 00:07:48,840 to give us a really basic idea of how routing works. 210 210 00:07:48,840 --> 00:07:51,070 Let's say that I'm sitting in router number five 211 211 00:07:51,070 --> 00:07:52,510 at the bottom-right corner, 212 212 00:07:52,510 --> 00:07:54,470 and I want to get to router number one. 213 213 00:07:54,470 --> 00:07:56,060 Well, how should I do it? 214 214 00:07:56,060 --> 00:07:59,340 I can go from five to four to one, and that would work, 215 215 00:07:59,340 --> 00:08:02,790 but I can also go from five to four to three to two to one, 216 216 00:08:02,790 --> 00:08:04,330 and that would also work. 217 217 00:08:04,330 --> 00:08:06,010 So, how do I know which way 218 218 00:08:06,010 --> 00:08:08,490 is going to be the best way for me to go? 219 219 00:08:08,490 --> 00:08:11,350 Well, if I end up using a dynamic protocol, 220 220 00:08:11,350 --> 00:08:13,180 all of these routers continually talk 221 221 00:08:13,180 --> 00:08:15,710 to each other all the time, and they tell each other 222 222 00:08:15,710 --> 00:08:17,960 which way they know how to get to other routers 223 223 00:08:17,960 --> 00:08:20,770 and which one is the best and fastest route. 224 224 00:08:20,770 --> 00:08:22,540 So, if you think about this like streets, 225 225 00:08:22,540 --> 00:08:24,280 when you type into your GPS 226 226 00:08:24,280 --> 00:08:26,090 that you want to go from point A to point B 227 227 00:08:26,090 --> 00:08:27,520 to get to the grocery store, 228 228 00:08:27,520 --> 00:08:29,750 it may take you three or four different ways 229 229 00:08:29,750 --> 00:08:31,950 depending on the time of day, the traffic, 230 230 00:08:31,950 --> 00:08:34,650 the congestion, and a number of other factors. 231 231 00:08:34,650 --> 00:08:37,080 Routers are doing the exact same thing. 232 232 00:08:37,080 --> 00:08:38,490 They all talk to each other and they say, 233 233 00:08:38,490 --> 00:08:40,300 hey, I've got a better way for you 234 234 00:08:40,300 --> 00:08:41,930 to get from point A to point B 235 235 00:08:41,930 --> 00:08:44,220 because there's too much traffic on this direction, 236 236 00:08:44,220 --> 00:08:46,580 so, you should go and take this other route, instead. 237 237 00:08:46,580 --> 00:08:49,920 That's the idea with route discovery and route selection. 238 238 00:08:49,920 --> 00:08:51,790 Now, the next thing we need to talk about here 239 239 00:08:51,790 --> 00:08:53,450 is connection services. 240 240 00:08:53,450 --> 00:08:55,630 And connection services are going to augment 241 241 00:08:55,630 --> 00:08:57,420 our layer 2 connection services 242 242 00:08:57,420 --> 00:08:58,830 that we talked about previously 243 243 00:08:58,830 --> 00:09:01,280 and provide us with some additional reliability. 244 244 00:09:01,280 --> 00:09:04,250 Again, we're going to have some more flow control added here, 245 245 00:09:04,250 --> 00:09:05,620 and this is going to prevent the sender 246 246 00:09:05,620 --> 00:09:08,660 from sending data faster than the receiver can get it. 247 247 00:09:08,660 --> 00:09:10,950 Again, that's the way that we have flow control there, 248 248 00:09:10,950 --> 00:09:13,300 so it can say, hey, hey, hey, slow down, 249 249 00:09:13,300 --> 00:09:14,980 you're sending me too much data, 250 250 00:09:14,980 --> 00:09:18,180 or speed up, I can take more, I'm ready for more. 251 251 00:09:18,180 --> 00:09:20,680 We also have this thing called packet reordering. 252 252 00:09:20,680 --> 00:09:22,960 Now, packet reordering is really important 253 253 00:09:22,960 --> 00:09:25,750 because it allows us to take this big chunk of data, 254 254 00:09:25,750 --> 00:09:27,730 cut it up into little pieces of packets, 255 255 00:09:27,730 --> 00:09:30,740 and then, send all those packets off in different directions 256 256 00:09:30,740 --> 00:09:32,550 to get to their final destination. 257 257 00:09:32,550 --> 00:09:34,940 Now, the problem is sometimes these packets 258 258 00:09:34,940 --> 00:09:37,970 are going to arrive at the destination in the wrong order. 259 259 00:09:37,970 --> 00:09:39,840 And so, packet reordering allows them 260 260 00:09:39,840 --> 00:09:43,480 to get all this data at the end destination at the receiver, 261 261 00:09:43,480 --> 00:09:44,890 and they can take and say, okay, 262 262 00:09:44,890 --> 00:09:47,800 I got packet one and packet five and packet two 263 263 00:09:47,800 --> 00:09:49,720 and packet four and packet three, 264 264 00:09:49,720 --> 00:09:51,500 and then, I'm going to put them in the right order, 265 265 00:09:51,500 --> 00:09:53,200 one, two, three, four, five, 266 266 00:09:53,200 --> 00:09:56,450 and then I can put that data back together in what it is, 267 267 00:09:56,450 --> 00:09:59,160 and now, I have the full piece of data together. 268 268 00:09:59,160 --> 00:10:01,290 The benefit here is that because of routing, 269 269 00:10:01,290 --> 00:10:03,680 each packet gets numbered and sequenced, 270 270 00:10:03,680 --> 00:10:06,330 and so, even if they get to the other end out of order, 271 271 00:10:06,330 --> 00:10:08,390 we can put them back into the right order 272 272 00:10:08,390 --> 00:10:10,790 and read them as a coherent message. 273 273 00:10:10,790 --> 00:10:12,890 The next thing we need to talk about here at layer 3 274 274 00:10:12,890 --> 00:10:17,180 is known as ICMP or the Internet Control Message Protocol. 275 275 00:10:17,180 --> 00:10:19,040 ICMP is used to send messages 276 276 00:10:19,040 --> 00:10:22,580 and operational information to an IP destination. 277 277 00:10:22,580 --> 00:10:26,780 The most commonly used one is known as ping, P-I-N-G. 278 278 00:10:26,780 --> 00:10:28,890 And we're going to talk specifically about that tool 279 279 00:10:28,890 --> 00:10:30,960 in our troubleshooting lecture. 280 280 00:10:30,960 --> 00:10:32,620 As you can see in this example, 281 281 00:10:32,620 --> 00:10:36,720 we can send out a single packet as a test to example.com. 282 282 00:10:36,720 --> 00:10:37,870 And when it comes back, 283 283 00:10:37,870 --> 00:10:40,250 we can then say if that site is up or down. 284 284 00:10:40,250 --> 00:10:41,560 This is what ping does. 285 285 00:10:41,560 --> 00:10:43,370 It sends out a packet and tells us 286 286 00:10:43,370 --> 00:10:45,350 if it was received or not by the distant end 287 287 00:10:45,350 --> 00:10:46,890 and how long it took. 288 288 00:10:46,890 --> 00:10:50,040 In this case, we got a response back five different times, 289 289 00:10:50,040 --> 00:10:51,530 showing that it was up 290 290 00:10:51,530 --> 00:10:53,770 and we were able to get to that distant end. 291 291 00:10:53,770 --> 00:10:55,870 Now, this is not a tool that's used regularly 292 292 00:10:55,870 --> 00:10:57,500 by end-user applications, 293 293 00:10:57,500 --> 00:10:59,620 but it is used by us as administrators 294 294 00:10:59,620 --> 00:11:01,650 to help troubleshoot our network and figure out 295 295 00:11:01,650 --> 00:11:04,760 what is up and what is down and what isn't working. 296 296 00:11:04,760 --> 00:11:07,890 Again, the most commonly used one here is going to be ping, 297 297 00:11:07,890 --> 00:11:11,135 but there's another variation of it known as traceroute, 298 298 00:11:11,135 --> 00:11:12,790 which will trace the route that a packet takes 299 299 00:11:12,790 --> 00:11:15,400 through the network and tells you every single router 300 300 00:11:15,400 --> 00:11:17,370 along the way as it goes through, 301 301 00:11:17,370 --> 00:11:19,620 essentially doing a large series of pings 302 302 00:11:19,620 --> 00:11:20,860 through each and every router 303 303 00:11:20,860 --> 00:11:22,600 so you could figure out which routes were up 304 304 00:11:22,600 --> 00:11:24,380 and which routes were down. 305 305 00:11:24,380 --> 00:11:26,910 Now, what are some examples of layer 3 devices 306 306 00:11:26,910 --> 00:11:28,820 that we need to remember for the exam? 307 307 00:11:28,820 --> 00:11:30,380 Well, the first two you have to remember 308 308 00:11:30,380 --> 00:11:32,910 are routers and multilayer switches. 309 309 00:11:32,910 --> 00:11:35,270 A router looks like this die icon here. 310 310 00:11:35,270 --> 00:11:36,550 You can see it's on the screen. 311 311 00:11:36,550 --> 00:11:39,370 It's a circle with four arrows, and this is a depiction 312 312 00:11:39,370 --> 00:11:42,210 of what a router looks like in a logical diagram. 313 313 00:11:42,210 --> 00:11:44,280 Now, a multilayer switch works 314 314 00:11:44,280 --> 00:11:46,720 like a regular switch and a router combined. 315 315 00:11:46,720 --> 00:11:49,200 So, it has both features of a layer 2 switch 316 316 00:11:49,200 --> 00:11:52,010 and a layer 3 router in the single device, 317 317 00:11:52,010 --> 00:11:54,420 which is why it's considered a layer 3 device. 318 318 00:11:54,420 --> 00:11:56,870 Again, for the exam, remember that a switch 319 319 00:11:56,870 --> 00:12:00,300 is always a layer 2 device unless they specifically tell you 320 320 00:12:00,300 --> 00:12:02,220 that it is a multilayer switch. 321 321 00:12:02,220 --> 00:12:03,640 If it's a multilayer switch, 322 322 00:12:03,640 --> 00:12:06,430 it is going to be considered a layer 3 device. 323 323 00:12:06,430 --> 00:12:08,110 Now, some other things that we have 324 324 00:12:08,110 --> 00:12:10,780 is going to be things like IPv4 and IPv6. 325 325 00:12:10,780 --> 00:12:13,080 These are both layer 3 protocols. 326 326 00:12:13,080 --> 00:12:16,230 We also have ICMP, the Internet Control Message Protocol, 327 327 00:12:16,230 --> 00:12:18,890 that we just talked about that's used in troubleshooting. 328 328 00:12:18,890 --> 00:12:21,520 All of these are found at layer 3. 329 329 00:12:21,520 --> 00:12:24,010 The best one to remember is IP and routers 330 330 00:12:24,010 --> 00:12:25,950 because these are going to be the most common ones 331 331 00:12:25,950 --> 00:12:27,510 you're going to see on test day 332 332 00:12:27,510 --> 00:12:30,423 if they ask you for examples of a layer 3 device.