1 00:00:00,000 --> 00:00:01,589 Now that we're at the network layer, 2 00:00:01,589 --> 00:00:03,420 we're concerned with routing. 3 00:00:03,420 --> 00:00:06,360 Layer three is all about how we're going to forward traffic, 4 00:00:06,360 --> 00:00:09,900 which we refer to as routing, using logical addresses. 5 00:00:09,900 --> 00:00:13,290 For example, your computer has an IP address, 6 00:00:13,290 --> 00:00:17,023 and that IP address is either going to be an IPv4 7 00:00:17,023 --> 00:00:19,560 or an IPv6 address, or both. 8 00:00:19,560 --> 00:00:22,470 Now, both of these are considered layer three protocols. 9 00:00:22,470 --> 00:00:23,700 And we're going to talk more about them 10 00:00:23,700 --> 00:00:25,680 as we go through this course. 11 00:00:25,680 --> 00:00:27,960 Now, the other thing we're going to be concerned with here is 12 00:00:27,960 --> 00:00:29,610 logical addressing. 13 00:00:29,610 --> 00:00:32,220 I mentioned that IPv4 and IPv6 are 14 00:00:32,220 --> 00:00:34,200 two types of logical addresses, 15 00:00:34,200 --> 00:00:36,720 but they're not the only logical addressing schemes 16 00:00:36,720 --> 00:00:37,710 that are out there. 17 00:00:37,710 --> 00:00:41,130 They're just the most common and most popular these days. 18 00:00:41,130 --> 00:00:42,510 Now, we're also going to be concerned 19 00:00:42,510 --> 00:00:44,250 with what's known as switching. 20 00:00:44,250 --> 00:00:45,900 And here when we talk about switching, 21 00:00:45,900 --> 00:00:48,510 we're actually talking about layer-three switching, 22 00:00:48,510 --> 00:00:49,980 which is called routing. 23 00:00:49,980 --> 00:00:51,360 Now, I know this gets confusing 24 00:00:51,360 --> 00:00:53,190 because they're using the term switching 25 00:00:53,190 --> 00:00:55,200 to refer to the function of routing, 26 00:00:55,200 --> 00:00:57,000 and when we talk about switches, 27 00:00:57,000 --> 00:00:59,310 the devices being layer two devices. 28 00:00:59,310 --> 00:01:00,900 So you have to keep this straight in your head. 29 00:01:00,900 --> 00:01:03,930 Switches, the physical device are layer two. 30 00:01:03,930 --> 00:01:06,360 Switching is another term for routing, 31 00:01:06,360 --> 00:01:07,950 which is how we transfer things 32 00:01:07,950 --> 00:01:10,770 at the network layer, layer three. 33 00:01:10,770 --> 00:01:12,450 Now, as we talk about all this, 34 00:01:12,450 --> 00:01:13,800 another thing we comes up is 35 00:01:13,800 --> 00:01:16,950 how we're going to do route discovery and selection. 36 00:01:16,950 --> 00:01:18,300 Now basically that means, 37 00:01:18,300 --> 00:01:21,330 how do I know which way I want the traffic to go? 38 00:01:21,330 --> 00:01:23,160 We're going to talk a little bit more about that too 39 00:01:23,160 --> 00:01:24,660 as we go through this lesson, 40 00:01:24,660 --> 00:01:26,850 because we're going to talk about connection services, 41 00:01:26,850 --> 00:01:29,700 and bandwidth utilization, and multiplexing strategies. 42 00:01:29,700 --> 00:01:32,250 All of this is at the layer three of the OC model. 43 00:01:32,250 --> 00:01:34,500 So to start diving in deeper into these concepts, 44 00:01:34,500 --> 00:01:36,990 let's start out with logical addresses. 45 00:01:36,990 --> 00:01:39,240 There are lots of different routed protocols 46 00:01:39,240 --> 00:01:41,040 that have been used over the years. 47 00:01:41,040 --> 00:01:42,750 Back in the '80s and '90s, 48 00:01:42,750 --> 00:01:45,270 there was AppleTalk for Apple computers. 49 00:01:45,270 --> 00:01:48,300 And if you used a Windows or a Novell network computer, 50 00:01:48,300 --> 00:01:49,890 you might have used IPX, 51 00:01:49,890 --> 00:01:52,470 which was the Internetwork Packet Exchange. 52 00:01:52,470 --> 00:01:54,240 Now, neither of these two are important 53 00:01:54,240 --> 00:01:56,160 for the Network+ certification, 54 00:01:56,160 --> 00:01:57,660 but it is something that I want to bring up 55 00:01:57,660 --> 00:01:59,760 because you may hear these terms. 56 00:01:59,760 --> 00:02:02,640 Really what happened was these were both killed off 57 00:02:02,640 --> 00:02:05,550 by internet protocol, which is known as IP. 58 00:02:05,550 --> 00:02:07,050 This became the common protocol 59 00:02:07,050 --> 00:02:09,120 that everyone uses on all networks. 60 00:02:09,120 --> 00:02:10,979 And therefore we didn't need AppleTalk 61 00:02:10,979 --> 00:02:12,960 and we didn't need IPX anymore. 62 00:02:12,960 --> 00:02:16,590 But the point is, at layer three, it's not just IP. 63 00:02:16,590 --> 00:02:17,760 There are other protocols 64 00:02:17,760 --> 00:02:20,010 that you could use in layer three. 65 00:02:20,010 --> 00:02:22,410 IP is just the most common. 66 00:02:22,410 --> 00:02:24,270 Now, some of those are still existing 67 00:02:24,270 --> 00:02:25,590 on some legacy systems, 68 00:02:25,590 --> 00:02:28,680 which means old systems in some corporate network, 69 00:02:28,680 --> 00:02:31,560 but the routing protocol of the internet that we use 70 00:02:31,560 --> 00:02:32,880 and the internet you have at home 71 00:02:32,880 --> 00:02:36,600 and the network you have at home is going to be known as IP. 72 00:02:36,600 --> 00:02:39,450 IP comes in two variants, as I said before, 73 00:02:39,450 --> 00:02:42,150 IPv4 and IPv6. 74 00:02:42,150 --> 00:02:43,860 Now, if you look on the screen here, 75 00:02:43,860 --> 00:02:46,110 there's an example of an IP address. 76 00:02:46,110 --> 00:02:50,673 It's written as 172.16.254.1. 77 00:02:52,140 --> 00:02:53,940 Now we're going to look more at IP addresses 78 00:02:53,940 --> 00:02:57,690 in a separate lesson as we dig deeper into routing later on. 79 00:02:57,690 --> 00:02:58,710 For the time being, 80 00:02:58,710 --> 00:03:00,840 I want you to think of an IP address 81 00:03:00,840 --> 00:03:03,060 anytime you see a number that looks like this. 82 00:03:03,060 --> 00:03:06,120 There's going to be four sets of numbers separated by dots. 83 00:03:06,120 --> 00:03:08,580 This is called a dotted octet notation. 84 00:03:08,580 --> 00:03:11,790 And this is what an IPv4 address is going to look like. 85 00:03:11,790 --> 00:03:14,550 Now, how should we actually forward or route the data 86 00:03:14,550 --> 00:03:15,870 across our networks? 87 00:03:15,870 --> 00:03:18,840 This is really the big question at layer three. 88 00:03:18,840 --> 00:03:21,330 And there are three main ways for us to do this. 89 00:03:21,330 --> 00:03:23,700 You can use packet switching, circuit switching, 90 00:03:23,700 --> 00:03:25,110 or message switching. 91 00:03:25,110 --> 00:03:27,150 The most commonly used one in your network 92 00:03:27,150 --> 00:03:28,350 is going to be routing, 93 00:03:28,350 --> 00:03:30,420 which is also known as packet switching. 94 00:03:30,420 --> 00:03:32,610 This is where data is divided into packets, 95 00:03:32,610 --> 00:03:34,710 and then each packet is forwarded on 96 00:03:34,710 --> 00:03:36,600 based on its IP address. 97 00:03:36,600 --> 00:03:37,740 Now, when I think of routing, 98 00:03:37,740 --> 00:03:40,410 I like to think about this as if I'm going to write a letter 99 00:03:40,410 --> 00:03:41,820 and send it to my mom. 100 00:03:41,820 --> 00:03:43,530 Let's say I put that letter in an envelope, 101 00:03:43,530 --> 00:03:45,060 and on the outside of the envelope, 102 00:03:45,060 --> 00:03:46,890 I'm going to write the address of my mom on it. 103 00:03:46,890 --> 00:03:49,260 I put her city and her state and the zip code. 104 00:03:49,260 --> 00:03:50,670 Now I put that in the mailbox, 105 00:03:50,670 --> 00:03:53,940 and the mail carrier is going to take it to a central location. 106 00:03:53,940 --> 00:03:56,340 Here, they're going to figure out what state it goes to. 107 00:03:56,340 --> 00:03:59,280 And then they're going to send it to that state's post office. 108 00:03:59,280 --> 00:04:01,830 Once it's in that state, it's going to go down even further. 109 00:04:01,830 --> 00:04:04,410 And they're going to go down to the city-level post office. 110 00:04:04,410 --> 00:04:06,810 And then from that city-level post office, 111 00:04:06,810 --> 00:04:08,850 they're going to look at where the street address is, 112 00:04:08,850 --> 00:04:09,930 get it to the right street, 113 00:04:09,930 --> 00:04:12,900 and eventually get it to her house on that street. 114 00:04:12,900 --> 00:04:16,110 The same kind of concept works with IP addresses. 115 00:04:16,110 --> 00:04:17,519 And that's the idea here. 116 00:04:17,519 --> 00:04:19,829 It's going to keep going and switching that packet 117 00:04:19,829 --> 00:04:23,100 from place to place until it gets to its final destination, 118 00:04:23,100 --> 00:04:25,890 in the case of my envelope to my mom's address. 119 00:04:25,890 --> 00:04:26,723 Now that's the way 120 00:04:26,723 --> 00:04:28,980 that packet switching is going to work for us. 121 00:04:28,980 --> 00:04:30,420 Every time I send a letter out, 122 00:04:30,420 --> 00:04:32,370 it might take a different route to get there. 123 00:04:32,370 --> 00:04:34,110 And I really don't care which route it takes 124 00:04:34,110 --> 00:04:36,720 as long as it gets to its final destination. 125 00:04:36,720 --> 00:04:39,120 It's the same thing with our packets in the network. 126 00:04:39,120 --> 00:04:41,190 When I talk about circuit switching though, 127 00:04:41,190 --> 00:04:43,260 this is where we want to have the same path 128 00:04:43,260 --> 00:04:44,670 each and every time. 129 00:04:44,670 --> 00:04:46,890 We're going to get a dedicated communication link 130 00:04:46,890 --> 00:04:48,930 that's established between our two devices. 131 00:04:48,930 --> 00:04:51,060 And if I pick up the phone to make a phone call, 132 00:04:51,060 --> 00:04:54,210 I'm actually going to make a virtual connection from my phone 133 00:04:54,210 --> 00:04:57,270 over to the other receiver's phone on the other end. 134 00:04:57,270 --> 00:04:58,980 So if I pick up the phone to call you, 135 00:04:58,980 --> 00:05:00,630 there's going to be a temporary connection made 136 00:05:00,630 --> 00:05:02,520 between my phone and your phone, 137 00:05:02,520 --> 00:05:03,390 and all the data 138 00:05:03,390 --> 00:05:05,130 that we're talking back and forth will go 139 00:05:05,130 --> 00:05:07,920 across the same path to get from me to you. 140 00:05:07,920 --> 00:05:09,630 The whole time we have this conversation going on, 141 00:05:09,630 --> 00:05:11,070 that's what's going to happen. 142 00:05:11,070 --> 00:05:13,830 That's what we call a circuit switch connection, 143 00:05:13,830 --> 00:05:16,380 which is different than the packet switch connection 144 00:05:16,380 --> 00:05:17,400 of using an envelope 145 00:05:17,400 --> 00:05:19,290 where we don't care where all those envelopes go 146 00:05:19,290 --> 00:05:21,690 as long as they got to the right place at the end. 147 00:05:21,690 --> 00:05:23,430 Now, when we hang up the phone 148 00:05:23,430 --> 00:05:24,810 and we go to make another call, 149 00:05:24,810 --> 00:05:26,190 it could take a different path. 150 00:05:26,190 --> 00:05:27,330 And that's okay. 151 00:05:27,330 --> 00:05:30,270 But for the entire session of us having our phone call, 152 00:05:30,270 --> 00:05:31,740 we want the same path. 153 00:05:31,740 --> 00:05:34,440 And that's what circuit switching allows us to do. 154 00:05:34,440 --> 00:05:36,780 Now, the third type of switching we have is known 155 00:05:36,780 --> 00:05:38,250 as message switching. 156 00:05:38,250 --> 00:05:39,390 And with message switching, 157 00:05:39,390 --> 00:05:42,030 this is where all the data is divided into messages. 158 00:05:42,030 --> 00:05:44,340 And they're similar to packet switching in this idea, 159 00:05:44,340 --> 00:05:47,520 but the messages can actually be stored and forwarded 160 00:05:47,520 --> 00:05:48,870 more like email. 161 00:05:48,870 --> 00:05:51,030 So if you go back to my mail example, 162 00:05:51,030 --> 00:05:52,560 maybe it gets to my mom's state, 163 00:05:52,560 --> 00:05:55,200 but the post office is closed because it's Sunday. 164 00:05:55,200 --> 00:05:56,190 So what happens is 165 00:05:56,190 --> 00:05:58,920 they drop all those envelopes on the floor. 166 00:05:58,920 --> 00:06:00,330 Now, it's going to be held there 167 00:06:00,330 --> 00:06:02,370 until they open again on Monday 168 00:06:02,370 --> 00:06:03,720 and somebody's going to be able to pick it up, 169 00:06:03,720 --> 00:06:06,060 figure out where it goes and push it along. 170 00:06:06,060 --> 00:06:07,020 This is what happens 171 00:06:07,020 --> 00:06:08,670 when you're dealing with message switching, 172 00:06:08,670 --> 00:06:11,160 because it has this store and forward capability. 173 00:06:11,160 --> 00:06:13,140 If we were using just packet switching, 174 00:06:13,140 --> 00:06:15,240 what would end up happening is if it got to the post office 175 00:06:15,240 --> 00:06:16,830 and the post office was closed, 176 00:06:16,830 --> 00:06:18,750 it would actually just shred that envelope 177 00:06:18,750 --> 00:06:20,190 and nobody would ever see it. 178 00:06:20,190 --> 00:06:21,023 That's a bad thing 179 00:06:21,023 --> 00:06:23,430 if we want to make sure the data's going to get where it's going. 180 00:06:23,430 --> 00:06:27,090 And that's why message switching can be very useful for us. 181 00:06:27,090 --> 00:06:29,100 Now, almost all of our networks nowadays 182 00:06:29,100 --> 00:06:31,200 and the ones you utilize are going to be using 183 00:06:31,200 --> 00:06:32,520 packet switching though. 184 00:06:32,520 --> 00:06:34,740 And the reason is we have other methods 185 00:06:34,740 --> 00:06:35,670 that will check 186 00:06:35,670 --> 00:06:37,800 if something is not getting to the distant end. 187 00:06:37,800 --> 00:06:39,690 And it'll be resend over another path 188 00:06:39,690 --> 00:06:41,280 until it finally gets there. 189 00:06:41,280 --> 00:06:42,120 So unless you're dealing 190 00:06:42,120 --> 00:06:44,310 with some kind of big backend networks, 191 00:06:44,310 --> 00:06:46,410 you're not really going to see something like circuit switching 192 00:06:46,410 --> 00:06:49,410 or message switching in your normal everyday networks. 193 00:06:49,410 --> 00:06:51,990 Your home network, my small office network, 194 00:06:51,990 --> 00:06:54,120 and most of the internet actually works 195 00:06:54,120 --> 00:06:55,470 using packet switching. 196 00:06:55,470 --> 00:06:57,480 Now, the second thing we have to talk about is 197 00:06:57,480 --> 00:06:59,370 route discovery and selection. 198 00:06:59,370 --> 00:07:01,800 How are we going to decide which path we're going to take 199 00:07:01,800 --> 00:07:03,540 to send that message? 200 00:07:03,540 --> 00:07:06,300 Well, routers are going to maintain a routing table 201 00:07:06,300 --> 00:07:08,370 so they can understand how to forward a packet 202 00:07:08,370 --> 00:07:11,520 based on the destination IP of where it wants to get to. 203 00:07:11,520 --> 00:07:13,680 There are lots of different ways that it can do this. 204 00:07:13,680 --> 00:07:15,720 And they can do this either as a static route 205 00:07:15,720 --> 00:07:18,600 or a dynamically assigned route using a routing protocol 206 00:07:18,600 --> 00:07:23,460 like RIP, OSPF, EIGRP, and many others. 207 00:07:23,460 --> 00:07:25,140 Now, we're going to talk about many of those 208 00:07:25,140 --> 00:07:26,640 later on in this course. 209 00:07:26,640 --> 00:07:27,473 So we're not going to talk 210 00:07:27,473 --> 00:07:29,730 specifically about how it works right now. 211 00:07:29,730 --> 00:07:31,440 We're just going to put that to the side, 212 00:07:31,440 --> 00:07:32,460 but I want you to remember 213 00:07:32,460 --> 00:07:34,920 that routing protocols help us decide 214 00:07:34,920 --> 00:07:37,350 how data is going to flow across the network 215 00:07:37,350 --> 00:07:40,290 and how the routers are going to communicate that information. 216 00:07:40,290 --> 00:07:42,660 For now, let's just use the example on the screen 217 00:07:42,660 --> 00:07:46,110 to give us a really basic idea of how routing works. 218 00:07:46,110 --> 00:07:48,330 Let's say that I'm sitting in router number five 219 00:07:48,330 --> 00:07:49,830 at the bottom right corner 220 00:07:49,830 --> 00:07:51,750 and I want to get to router number one. 221 00:07:51,750 --> 00:07:53,340 Well, how should I do it? 222 00:07:53,340 --> 00:07:56,640 I can go from five to four to one, and that would work. 223 00:07:56,640 --> 00:08:00,180 But I can also go from five to four to three to two to one, 224 00:08:00,180 --> 00:08:01,620 and that would also work. 225 00:08:01,620 --> 00:08:04,290 So how do I know which way is going to be the best way 226 00:08:04,290 --> 00:08:05,760 for me to go? 227 00:08:05,760 --> 00:08:08,640 Well, if I end up using a dynamic protocol, 228 00:08:08,640 --> 00:08:10,860 all of these routers continually talk to each other 229 00:08:10,860 --> 00:08:11,850 all the time, 230 00:08:11,850 --> 00:08:13,980 and they tell each other which way they know 231 00:08:13,980 --> 00:08:15,270 how to get to other routers 232 00:08:15,270 --> 00:08:18,060 and which one is the best and fastest route. 233 00:08:18,060 --> 00:08:19,830 So if you think about this like streets, 234 00:08:19,830 --> 00:08:21,570 when you type into your GPS 235 00:08:21,570 --> 00:08:23,400 that you want to go from point A to point B 236 00:08:23,400 --> 00:08:24,840 to get to the grocery store, 237 00:08:24,840 --> 00:08:27,030 it may take you three or four different ways 238 00:08:27,030 --> 00:08:30,000 depending on the time of day, the traffic, the congestion, 239 00:08:30,000 --> 00:08:31,950 and a number of other factors. 240 00:08:31,950 --> 00:08:34,500 Routers are doing the exact same thing. 241 00:08:34,500 --> 00:08:35,677 All talk to each other and say, 242 00:08:35,677 --> 00:08:37,590 "Hey, I've got a better way for you 243 00:08:37,590 --> 00:08:39,210 to get from point A to point B 244 00:08:39,210 --> 00:08:41,490 because there's too much traffic on this direction. 245 00:08:41,490 --> 00:08:43,860 So you should go and take this other route instead." 246 00:08:43,860 --> 00:08:47,220 That's the idea with route discovery and route selection. 247 00:08:47,220 --> 00:08:49,230 Now, the next thing we need to talk about here is 248 00:08:49,230 --> 00:08:50,790 connection services. 249 00:08:50,790 --> 00:08:52,920 And connection services are going to augment 250 00:08:52,920 --> 00:08:54,690 our layer-two connection services 251 00:08:54,690 --> 00:08:56,100 that we talked about previously 252 00:08:56,100 --> 00:08:58,560 and provide us with some additional reliability. 253 00:08:58,560 --> 00:09:01,530 Again, we're going to have some more flow control added here. 254 00:09:01,530 --> 00:09:02,880 And this is going to prevent the sender 255 00:09:02,880 --> 00:09:05,970 from sending data faster than the receiver can get it. 256 00:09:05,970 --> 00:09:08,250 Again, that's the way that we have flow control there, 257 00:09:08,250 --> 00:09:09,083 so it can say, 258 00:09:09,083 --> 00:09:10,590 "Hey, hey, hey, slow down. 259 00:09:10,590 --> 00:09:12,270 You're sending me too much data," 260 00:09:12,270 --> 00:09:15,480 or, "Speed up, I can take more. I'm ready for more." 261 00:09:15,480 --> 00:09:17,970 We also have this thing called packet reordering. 262 00:09:17,970 --> 00:09:20,280 Now, packet reordering is really important 263 00:09:20,280 --> 00:09:23,040 because it allows us to take this big chunk of data, 264 00:09:23,040 --> 00:09:25,080 cut it up into little pieces of packets, 265 00:09:25,080 --> 00:09:28,020 and then send all those packets off in different directions 266 00:09:28,020 --> 00:09:29,820 to get to their final destination. 267 00:09:29,820 --> 00:09:30,960 Now, the problem is 268 00:09:30,960 --> 00:09:33,120 sometimes these packets are going to arrive 269 00:09:33,120 --> 00:09:35,250 at the destination in the wrong order. 270 00:09:35,250 --> 00:09:38,220 And so packet reordering allows them to get all this data 271 00:09:38,220 --> 00:09:40,770 at the end destination, at the receiver, 272 00:09:40,770 --> 00:09:41,603 and they can take and say, 273 00:09:41,603 --> 00:09:45,120 "Okay, I got packet one and packet five and packet two 274 00:09:45,120 --> 00:09:47,010 and packet four and packet three. 275 00:09:47,010 --> 00:09:48,780 and then I'm going to put them in the right order. 276 00:09:48,780 --> 00:09:50,520 One, two, three, four, five, 277 00:09:50,520 --> 00:09:53,730 and then I can put that data back together in what it is. 278 00:09:53,730 --> 00:09:56,460 And now I have the full piece of data together." 279 00:09:56,460 --> 00:09:58,590 The benefit here is that because of routing, 280 00:09:58,590 --> 00:10:00,990 each packet gets numbered and sequenced, 281 00:10:00,990 --> 00:10:03,630 and so even if they get to the other end out of order, 282 00:10:03,630 --> 00:10:05,670 we can put them back into the right order 283 00:10:05,670 --> 00:10:08,100 and read them as a coherent message. 284 00:10:08,100 --> 00:10:09,480 The next thing we need to talk about here 285 00:10:09,480 --> 00:10:11,820 at layer three is known as ICMP, 286 00:10:11,820 --> 00:10:14,460 or the Internet Control Message Protocol. 287 00:10:14,460 --> 00:10:16,350 ICMP is used to send messages 288 00:10:16,350 --> 00:10:19,890 and operational information to an IP destination. 289 00:10:19,890 --> 00:10:24,060 The most commonly used one is known as ping, P-I-N-G. 290 00:10:24,060 --> 00:10:26,160 And we're going to talk specifically about that tool 291 00:10:26,160 --> 00:10:28,230 in our troubleshooting lecture. 292 00:10:28,230 --> 00:10:29,880 As you can see in this example, 293 00:10:29,880 --> 00:10:33,990 we can send out a single packet as a test to example.com. 294 00:10:33,990 --> 00:10:35,160 And when it comes back, 295 00:10:35,160 --> 00:10:37,530 we can then say if that site is up or down. 296 00:10:37,530 --> 00:10:38,820 This is what ping does. 297 00:10:38,820 --> 00:10:39,960 It sends out a packet 298 00:10:39,960 --> 00:10:42,690 and tells us if it was received or not by the distant end 299 00:10:42,690 --> 00:10:44,160 and how long it took. 300 00:10:44,160 --> 00:10:44,993 In this case, 301 00:10:44,993 --> 00:10:47,340 we got a response back five different times 302 00:10:47,340 --> 00:10:48,810 showing that it was up 303 00:10:48,810 --> 00:10:51,060 and we were able to get to that distant end. 304 00:10:51,060 --> 00:10:53,160 Now, this is not a tool that's used regularly 305 00:10:53,160 --> 00:10:54,780 by end user applications, 306 00:10:54,780 --> 00:10:56,880 but it is used by us as administrators 307 00:10:56,880 --> 00:10:58,920 to help troubleshoot our network and figure out 308 00:10:58,920 --> 00:11:00,450 what is up, and what is down, 309 00:11:00,450 --> 00:11:02,040 and what isn't working. 310 00:11:02,040 --> 00:11:05,190 Again, the most commonly used one here is going to be ping, 311 00:11:05,190 --> 00:11:08,010 but there's another variation of it known as traceroute, 312 00:11:08,010 --> 00:11:10,080 which will trace the route that a packet takes 313 00:11:10,080 --> 00:11:11,070 through the network 314 00:11:11,070 --> 00:11:13,440 and tells you every single router along the way 315 00:11:13,440 --> 00:11:14,670 as it goes through, 316 00:11:14,670 --> 00:11:16,920 essentially doing a large series of pings 317 00:11:16,920 --> 00:11:18,150 through each and every router 318 00:11:18,150 --> 00:11:19,890 so you can figure out which routes we're up 319 00:11:19,890 --> 00:11:21,330 and which routes we're down. 320 00:11:21,330 --> 00:11:23,880 Now, what are some examples of layer-three devices 321 00:11:23,880 --> 00:11:25,740 that we need to remember for the exam? 322 00:11:25,740 --> 00:11:27,540 Well, the first two you have to remember are 323 00:11:27,540 --> 00:11:29,820 routers and multi-layer switches. 324 00:11:29,820 --> 00:11:31,920 Router looks like this icon here. 325 00:11:31,920 --> 00:11:33,180 You can see it's on the screen. 326 00:11:33,180 --> 00:11:35,010 It's a circle with four arrows. 327 00:11:35,010 --> 00:11:37,050 And this is a depiction of what a router looks like 328 00:11:37,050 --> 00:11:38,820 in a logical diagram. 329 00:11:38,820 --> 00:11:41,850 Now, a multi-layer switch works like a regular switch 330 00:11:41,850 --> 00:11:43,350 and our router combined. 331 00:11:43,350 --> 00:11:45,840 So it has both features of a layer-two switch 332 00:11:45,840 --> 00:11:48,630 and a layer-three router in the single device, 333 00:11:48,630 --> 00:11:51,030 which is why it's considered a layer-three device. 334 00:11:51,030 --> 00:11:52,380 Again, for the exam, 335 00:11:52,380 --> 00:11:55,230 remember that a switch is always a layer-two device 336 00:11:55,230 --> 00:11:56,940 unless they specifically tell you 337 00:11:56,940 --> 00:11:58,830 that it is a multi-layer switch. 338 00:11:58,830 --> 00:12:00,270 If it's a multi-layer switch, 339 00:12:00,270 --> 00:12:03,030 it is going to be considered a layer-three device. 340 00:12:03,030 --> 00:12:05,370 Now, some other things that we have is going to be things 341 00:12:05,370 --> 00:12:07,410 like IPv4 and IPv6. 342 00:12:07,410 --> 00:12:09,690 These are both layer-three protocols. 343 00:12:09,690 --> 00:12:12,840 We also have ICMP, the Internet Control Message Protocol, 344 00:12:12,840 --> 00:12:15,480 that we just talked about that's used in troubleshooting. 345 00:12:15,480 --> 00:12:18,150 All of these are found at layer three. 346 00:12:18,150 --> 00:12:20,610 The best one to remember is IP and routers, 347 00:12:20,610 --> 00:12:22,560 because these are going to be the most common ones 348 00:12:22,560 --> 00:12:24,120 you're going to see on test A 349 00:12:24,120 --> 00:12:27,003 if they ask you for examples of a layer-three device.