1 00:00:00,006 --> 00:00:01,009 - [Instructor] When working with addressing 2 00:00:01,009 --> 00:00:04,008 and information flow, I tend to spend 3 00:00:04,008 --> 00:00:08,002 a significant amount of time in Layers 2 and 3, 4 00:00:08,002 --> 00:00:11,000 which are data-link and network respectively. 5 00:00:11,000 --> 00:00:14,003 Layer 2, the data-link layer, refers to communication 6 00:00:14,003 --> 00:00:17,007 between devices on a common switched infrastructure. 7 00:00:17,007 --> 00:00:21,003 This layer is best known for switches and switching. 8 00:00:21,003 --> 00:00:25,009 Each device uses a media access control or MAC address. 9 00:00:25,009 --> 00:00:29,007 MAC addresses are six groups of two hexadecimal digits, 10 00:00:29,007 --> 00:00:32,000 usually separated by colons or hyphens, 11 00:00:32,000 --> 00:00:34,005 but sometimes without a separator. 12 00:00:34,005 --> 00:00:36,005 This layer deals only with switching 13 00:00:36,005 --> 00:00:38,004 so frames don't really care 14 00:00:38,004 --> 00:00:42,008 about the encapsulated IP addresses they are transferring. 15 00:00:42,008 --> 00:00:47,004 Layer 3 refers to sending information via IP addresses. 16 00:00:47,004 --> 00:00:51,000 This layer is best known for routers and routing. 17 00:00:51,000 --> 00:00:53,006 As the information passes from router to router 18 00:00:53,006 --> 00:00:57,005 on its way to a destination, it will use Layer 3 information 19 00:00:57,005 --> 00:00:59,003 to see where to forward the packet, 20 00:00:59,003 --> 00:01:03,000 then repeat the process until it reaches its destination. 21 00:01:03,000 --> 00:01:05,009 As information moves up and down the OSI Model, 22 00:01:05,009 --> 00:01:08,009 some layers have a special name for the data 23 00:01:08,009 --> 00:01:10,007 at one of those layers. 24 00:01:10,007 --> 00:01:13,008 This is called a Protocol Data Unit. 25 00:01:13,008 --> 00:01:15,006 As this information moves down, 26 00:01:15,006 --> 00:01:19,000 it is encapsulated into the lower PDU. 27 00:01:19,000 --> 00:01:22,006 As it arrives to a host and moves up the OSI Model, 28 00:01:22,006 --> 00:01:25,000 it is de-capsulated. 29 00:01:25,000 --> 00:01:27,005 Think of it as Russian nesting dolls, 30 00:01:27,005 --> 00:01:30,002 enclosing one PDU within another. 31 00:01:30,002 --> 00:01:32,006 While I'm focusing on Layer 2 and 3, 32 00:01:32,006 --> 00:01:34,003 all layers are very important 33 00:01:34,003 --> 00:01:36,003 to the troubleshooting process. 34 00:01:36,003 --> 00:01:39,004 Layer 1's PDU is known as a bit. 35 00:01:39,004 --> 00:01:41,001 Since many of networking's issues 36 00:01:41,001 --> 00:01:45,002 can be traced to a physical issue, it must not be forgotten. 37 00:01:45,002 --> 00:01:48,001 Moving to Layer 2, we have frames. 38 00:01:48,001 --> 00:01:49,009 Frames are the last PDUs used 39 00:01:49,009 --> 00:01:52,007 before information is put on the wire for transmission 40 00:01:52,007 --> 00:01:55,008 or the first received off of the wire. 41 00:01:55,008 --> 00:01:59,004 A frame consists of bits arranged as framing bits, 42 00:01:59,004 --> 00:02:02,005 packet payload, and the frame check sequence. 43 00:02:02,005 --> 00:02:04,005 Framing bits are used in conjunction 44 00:02:04,005 --> 00:02:06,007 with a Layer 2 transmission method. 45 00:02:06,007 --> 00:02:09,001 The FCS is an extra bit of code 46 00:02:09,001 --> 00:02:11,003 put in place for error detection. 47 00:02:11,003 --> 00:02:14,008 The PDUs at Layer 3 are packets. 48 00:02:14,008 --> 00:02:18,004 Packets are formed of control information and the payload. 49 00:02:18,004 --> 00:02:20,005 Control information will consist of things 50 00:02:20,005 --> 00:02:24,006 like source and destination IP, sequence information, 51 00:02:24,006 --> 00:02:26,007 and error detection information. 52 00:02:26,007 --> 00:02:30,005 The payload consists of user data to be transported. 53 00:02:30,005 --> 00:02:35,005 Layer 4 consists of TCP segments or UDP datagrams. 54 00:02:35,005 --> 00:02:39,005 The TCP segment consists of a header and a data portion. 55 00:02:39,005 --> 00:02:41,000 The header includes information 56 00:02:41,000 --> 00:02:43,006 like source and destination IP, 57 00:02:43,006 --> 00:02:48,007 sequence number, window size, port numbers, and so on. 58 00:02:48,007 --> 00:02:51,008 The UDP datagram is designed in much the same way, 59 00:02:51,008 --> 00:02:54,002 but the header contains fewer options, 60 00:02:54,002 --> 00:02:57,001 though it does retain the key pieces. 61 00:02:57,001 --> 00:03:00,002 Layer 5 through 7 is just considered data. 62 00:03:00,002 --> 00:03:03,008 It can be compressed, encrypted, or clear text. 63 00:03:03,008 --> 00:03:06,005 Common nomenclature is important when troubleshooting 64 00:03:06,005 --> 00:03:09,008 or simply having a functionality discussion. 65 00:03:09,008 --> 00:03:13,000 Be sure to add the various PDUs to your vernacular.