1 00:00:01,192 --> 00:00:03,030 The OSI Model. 2 00:00:03,030 --> 00:00:04,860 The Open Systems Interconnection 3 00:00:04,860 --> 00:00:08,380 or OSI Model is used to explain how network communications 4 00:00:08,380 --> 00:00:10,820 occur between a host and a remote device 5 00:00:10,820 --> 00:00:13,760 over a local area network or a LAN. 6 00:00:13,760 --> 00:00:16,560 Now, the OSI Model is very useful to help us 7 00:00:16,560 --> 00:00:19,070 categorize different communication protocols 8 00:00:19,070 --> 00:00:20,050 that are used in networks, 9 00:00:20,050 --> 00:00:22,410 and gives us a common lexicon that we can use 10 00:00:22,410 --> 00:00:25,720 to describe the functions of different devices. 11 00:00:25,720 --> 00:00:27,890 Now, going back to your Network+ studies, 12 00:00:27,890 --> 00:00:29,470 you probably remember the mnemonic 13 00:00:29,470 --> 00:00:32,590 of Please Do Not Throw Sausage Pizza Away. 14 00:00:32,590 --> 00:00:35,550 This represents the seven layers of the OSI Model, 15 00:00:35,550 --> 00:00:37,430 going from the bottom to the top. 16 00:00:37,430 --> 00:00:41,440 This is: Physical, Data Link, Network, Transport, 17 00:00:41,440 --> 00:00:44,281 Session, Presentation, and Application. 18 00:00:44,281 --> 00:00:46,870 The first layer is the physical Layer. 19 00:00:46,870 --> 00:00:48,130 This is the layer that represents 20 00:00:48,130 --> 00:00:50,081 the actual network cables and radio waves 21 00:00:50,081 --> 00:00:52,820 that are used to carry data over a network. 22 00:00:52,820 --> 00:00:54,960 Data carried over the network at the Physical Layer 23 00:00:54,960 --> 00:00:56,230 is known as bits. 24 00:00:56,230 --> 00:00:59,470 And they can be electrical signal or radio wave. 25 00:00:59,470 --> 00:01:01,350 Examples of some of the things that operate 26 00:01:01,350 --> 00:01:03,230 at the Layer One or Physical Layer 27 00:01:03,230 --> 00:01:04,860 are the things like our network cables 28 00:01:04,860 --> 00:01:08,290 whether they're fiber optic, or copper, or coaxial. 29 00:01:08,290 --> 00:01:11,300 It could be radio waves like Wi-Fi and Bluetooth. 30 00:01:11,300 --> 00:01:13,200 It can be a hub or repeater, 31 00:01:13,200 --> 00:01:15,153 which are dumb devices that simply take inputs in 32 00:01:15,153 --> 00:01:18,090 and then repeat them out the other side. 33 00:01:18,090 --> 00:01:20,490 Our second layer is the Data Link Layer. 34 00:01:20,490 --> 00:01:22,410 This is the layer that describes how a connection 35 00:01:22,410 --> 00:01:24,820 is established, maintained, and transferred 36 00:01:24,820 --> 00:01:26,600 over that physical layer. 37 00:01:26,600 --> 00:01:29,370 Addressing here is done using physical addresses, 38 00:01:29,370 --> 00:01:31,060 like MAC addresses. 39 00:01:31,060 --> 00:01:33,380 Now, at this layer, the bits are going to be grouped 40 00:01:33,380 --> 00:01:36,150 into frames and then sent over the network. 41 00:01:36,150 --> 00:01:38,520 Examples of some things that operate at Layer Two 42 00:01:38,520 --> 00:01:41,410 or the Data Link Layer includes things like MAC addresses, 43 00:01:41,410 --> 00:01:43,080 switches, and bridges. 44 00:01:43,080 --> 00:01:44,970 Now, bridges are an earlier device 45 00:01:44,970 --> 00:01:47,480 that have later on evolved into switches. 46 00:01:47,480 --> 00:01:49,850 Switches use MAC addresses as their form 47 00:01:49,850 --> 00:01:51,200 of physical addressing. 48 00:01:51,200 --> 00:01:52,580 This allows a switch to decide 49 00:01:52,580 --> 00:01:54,715 where to send that frame of information 50 00:01:54,715 --> 00:01:57,510 based on the MAC address it's designed to go to. 51 00:01:57,510 --> 00:01:59,240 And so, it's smarter than a hub 52 00:01:59,240 --> 00:02:01,260 because it will decide where that particular 53 00:02:01,260 --> 00:02:03,340 frame goes as opposed to just repeating it out 54 00:02:03,340 --> 00:02:05,250 every single port that it has. 55 00:02:05,250 --> 00:02:07,970 Now, as we move up the ladder, we get to the third layer 56 00:02:07,970 --> 00:02:09,560 which is the Network Layer. 57 00:02:09,560 --> 00:02:11,650 This is the layer where logical addressing 58 00:02:11,650 --> 00:02:12,750 is actually performed. 59 00:02:12,750 --> 00:02:14,730 And this includes things like routing and switching 60 00:02:14,730 --> 00:02:17,120 information between hosts, the network, 61 00:02:17,120 --> 00:02:18,371 and the internetworks. 62 00:02:18,371 --> 00:02:21,560 At this layer, the frames are now taken and grouped up 63 00:02:21,560 --> 00:02:26,510 into packets, so bits became frames, frames become packets. 64 00:02:26,510 --> 00:02:28,400 Now, examples of this include things like 65 00:02:28,400 --> 00:02:30,800 the addresses which are IP addresses. 66 00:02:30,800 --> 00:02:33,590 This allows us to tell where a piece of information, 67 00:02:33,590 --> 00:02:36,360 where a packet is going to be sent over our network. 68 00:02:36,360 --> 00:02:38,590 We also use things like Layer Three switches, 69 00:02:38,590 --> 00:02:40,490 which in addition to using MAC addresses 70 00:02:40,490 --> 00:02:41,810 to decide where things go, 71 00:02:41,810 --> 00:02:44,560 they can use the IP addresses at Layer Three. 72 00:02:44,560 --> 00:02:46,342 And of course routers which are by far 73 00:02:46,342 --> 00:02:48,900 the most common Layer Three device, 74 00:02:48,900 --> 00:02:50,850 because they're used to connect all of our networks 75 00:02:50,850 --> 00:02:52,233 together around the world. 76 00:02:52,233 --> 00:02:54,777 Our fourth layer is the Transport Layer. 77 00:02:54,777 --> 00:02:57,390 This layer manages and ensures transmission 78 00:02:57,390 --> 00:02:59,520 of the packets occur from the host 79 00:02:59,520 --> 00:03:01,340 to the destination it wants. 80 00:03:01,340 --> 00:03:03,020 This uses either a TCP, 81 00:03:03,020 --> 00:03:04,930 known as a Connection Full Protocol 82 00:03:04,930 --> 00:03:07,880 or UDP which is a Connection Less Protocol. 83 00:03:07,880 --> 00:03:09,610 You might remember from Network+, 84 00:03:09,610 --> 00:03:11,700 TCP has that three-way handshake, 85 00:03:11,700 --> 00:03:13,930 and it says, hey I'm ready to send you something, 86 00:03:13,930 --> 00:03:15,540 okay I'm ready to be sent something. 87 00:03:15,540 --> 00:03:16,920 All right, let's start sending it. 88 00:03:16,920 --> 00:03:18,650 And then they send the information. 89 00:03:18,650 --> 00:03:19,930 With UDP, on the other hand, 90 00:03:19,930 --> 00:03:21,200 it's just fire and forget, 91 00:03:21,200 --> 00:03:22,033 we send a bunch of information 92 00:03:22,033 --> 00:03:24,380 and we just hope it gets there. 93 00:03:24,380 --> 00:03:26,510 At this layer, our packets have now 94 00:03:26,510 --> 00:03:29,110 been grouped into segments if we're using TCP 95 00:03:29,110 --> 00:03:31,000 or they're packaged into datagrams 96 00:03:31,000 --> 00:03:32,900 if we're using UDP. 97 00:03:32,900 --> 00:03:36,100 Next, we have the fifth layer which is our Session Layer. 98 00:03:36,100 --> 00:03:38,670 This layer manages the establishment, termination, 99 00:03:38,670 --> 00:03:41,732 and synchronization of a session over the network. 100 00:03:41,732 --> 00:03:44,510 This creates a unique connection over the network 101 00:03:44,510 --> 00:03:47,250 that allows data to find its way from the original host 102 00:03:47,250 --> 00:03:50,740 over to the destination host in a continual sequence. 103 00:03:50,740 --> 00:03:53,010 I like to think about this like making a phone call. 104 00:03:53,010 --> 00:03:55,350 When I pick up the phone and I dial your phone number, 105 00:03:55,350 --> 00:03:57,760 I don't really care which path my voice takes 106 00:03:57,760 --> 00:04:00,210 to get from me to you, which countries it goes through, 107 00:04:00,210 --> 00:04:01,770 or which cities it goes through. 108 00:04:01,770 --> 00:04:03,940 All I care about is that we have a good connection 109 00:04:03,940 --> 00:04:05,860 and that we can talk the entire time. 110 00:04:05,860 --> 00:04:07,800 That is creating a session. 111 00:04:07,800 --> 00:04:10,800 The sixth layer is what's known as the Presentation Layer. 112 00:04:10,800 --> 00:04:13,130 This layer is focused on translating the information 113 00:04:13,130 --> 00:04:16,040 into a format that both the sender and the receiver 114 00:04:16,040 --> 00:04:17,460 are going to understand. 115 00:04:17,460 --> 00:04:20,200 For example, if I send a message to you in text, 116 00:04:20,200 --> 00:04:23,608 you have to know if it's encoded in ASCII or UTF-8. 117 00:04:23,608 --> 00:04:25,950 Depending on which one it is, you're going to read that 118 00:04:25,950 --> 00:04:28,530 information differently, because at heart, all the 119 00:04:28,530 --> 00:04:31,270 sender and receiver are sending are ones and zeroes, right? 120 00:04:31,270 --> 00:04:33,290 And so, those ones and zeroes have to be gone 121 00:04:33,290 --> 00:04:37,130 and translated into a format that we both understand. 122 00:04:37,130 --> 00:04:39,790 Other examples of this might be things like JPGs 123 00:04:39,790 --> 00:04:42,560 or PNG files, or even using encryption. 124 00:04:42,560 --> 00:04:45,280 All of this happens at the Presentation Layer. 125 00:04:45,280 --> 00:04:47,088 It's how are we going to format those ones and zeroes, 126 00:04:47,088 --> 00:04:48,870 and understand how it's going to be displayed 127 00:04:48,870 --> 00:04:51,320 so the end-user can see it. 128 00:04:51,320 --> 00:04:52,640 The seventh and final layer 129 00:04:52,640 --> 00:04:54,620 is known as the Application Layer. 130 00:04:54,620 --> 00:04:56,070 This layer is where the message 131 00:04:56,070 --> 00:04:58,420 is originally created and sent from. 132 00:04:58,420 --> 00:05:01,380 Now, this isn't an application like Word, or PowerPoint, 133 00:05:01,380 --> 00:05:03,480 or Excel but instead, we're talking about 134 00:05:03,480 --> 00:05:07,570 high-level protocols, things like HTTP for web pages, 135 00:05:07,570 --> 00:05:11,860 SMTP for sending email, and FTP for file transfer. 136 00:05:11,860 --> 00:05:13,680 The application layer is where the user 137 00:05:13,680 --> 00:05:15,768 is really starting to interact with the network 138 00:05:15,768 --> 00:05:18,190 by using the OSI Model. 139 00:05:18,190 --> 00:05:19,610 Now, I know I went through all 140 00:05:19,610 --> 00:05:21,130 of this really, really quickly. 141 00:05:21,130 --> 00:05:23,730 And I hope this wasn't brand new information for you. 142 00:05:23,730 --> 00:05:26,090 Again at Security+, they're going to already 143 00:05:26,090 --> 00:05:28,820 assume that you've taken A+ and Network+. 144 00:05:28,820 --> 00:05:31,290 Even if you haven't passed those certification exams, 145 00:05:31,290 --> 00:05:32,160 they're expecting that you've 146 00:05:32,160 --> 00:05:33,360 gone through that curriculum 147 00:05:33,360 --> 00:05:36,930 because it's assumed knowledge for Security+. 148 00:05:36,930 --> 00:05:38,630 With Security+, we're looking at how we can start 149 00:05:38,630 --> 00:05:40,800 securing our networks from attack 150 00:05:40,800 --> 00:05:42,200 and our devices from attack. 151 00:05:42,200 --> 00:05:44,090 And those are all things that we talked about 152 00:05:44,090 --> 00:05:46,750 earlier on in A+ and Network+. 153 00:05:46,750 --> 00:05:48,369 So, if this was all brand new to you, 154 00:05:48,369 --> 00:05:51,230 I do recommend taking a short break here, 155 00:05:51,230 --> 00:05:53,930 going back and watching a Network+ video series 156 00:05:53,930 --> 00:05:55,920 or an A+ video series to make sure 157 00:05:55,920 --> 00:05:57,333 you have that foundational knowledge 158 00:05:57,333 --> 00:05:58,900 as we continue forward 159 00:05:58,900 --> 00:06:02,567 through our Security+ studies together.