1 00:00:00,330 --> 00:00:03,960 ‫So let's see what happens to a packet inside the network traffic. 2 00:00:04,860 --> 00:00:11,760 ‫Please note, the data unit transferred between two endpoints has different names in each layer and 3 00:00:11,760 --> 00:00:14,180 ‫layers five, six and seven application layers. 4 00:00:14,190 --> 00:00:18,000 ‫It's called data in layer for transport layer. 5 00:00:18,030 --> 00:00:24,840 ‫It's called Segment four TCP and data gram for UDP in layer three the network layer. 6 00:00:25,110 --> 00:00:28,280 ‫It's called packet in layer two. 7 00:00:28,350 --> 00:00:29,280 ‫The data link layer. 8 00:00:29,520 --> 00:00:31,620 ‫The data unit is called frame. 9 00:00:32,340 --> 00:00:38,970 ‫Now I usually use packet for the transferred data unit in each layer to make it less complicated for 10 00:00:38,970 --> 00:00:39,180 ‫you. 11 00:00:39,330 --> 00:00:43,770 ‫Makes sense before looking at the road trip of a DNS query in detail. 12 00:00:44,010 --> 00:00:48,810 ‫Let's look at the relationship between the osai layers and computer systems. 13 00:00:49,880 --> 00:00:55,490 ‫The packets and layer one and layer two are managed by network interfaces of your device, for example, 14 00:00:55,490 --> 00:00:56,810 ‫by your Ethernet card. 15 00:00:57,670 --> 00:01:03,190 ‫And the package from three to seven are managed by the processing unit in software detail. 16 00:01:04,160 --> 00:01:09,890 ‫The layer three and layer for packets are managed by the operating system of your devices, and the 17 00:01:09,890 --> 00:01:16,100 ‫packets of layer five to seven are managed by the related application or service, for example, a web 18 00:01:16,100 --> 00:01:16,610 ‫browser. 19 00:01:17,210 --> 00:01:20,600 ‫When you write a, you are URL in the address bar of your browser and hit enter. 20 00:01:21,670 --> 00:01:28,210 ‫The first thing it sends is a DNS query, a DNS query is the process of a computer networking device 21 00:01:28,450 --> 00:01:34,750 ‫making an enquiry to get an IP address for a DNS names such as Mail Dot Yahoo.com. 22 00:01:36,160 --> 00:01:42,130 ‫The client computer will send a DNS query to one of their internet service providers DNS servers. 23 00:01:42,790 --> 00:01:49,990 ‫The DNS server looks in its DNS database to tell whether it can answer the query authoritatively. 24 00:01:50,870 --> 00:01:57,740 ‫If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process 25 00:01:57,860 --> 00:01:58,620 ‫is complete. 26 00:01:59,360 --> 00:02:05,390 ‫So let's see the road trip of a DNS query from your computer to a DNS server when data is transmitted 27 00:02:05,390 --> 00:02:07,940 ‫by the source toward a specific destination. 28 00:02:08,720 --> 00:02:14,690 ‫It passes through the application presentation and the session layers, and the protocol data unit arrives 29 00:02:14,690 --> 00:02:15,860 ‫at the transport layer. 30 00:02:16,130 --> 00:02:22,430 ‫Layer four layers five, six and seven are displayed as a single layer to simplify the presentation. 31 00:02:22,970 --> 00:02:27,410 ‫At this layer, a 20 byte data header is placed in front of the data. 32 00:02:27,890 --> 00:02:30,330 ‫A DNS query can also use TCP. 33 00:02:30,590 --> 00:02:35,870 ‫But let's assume that it uses UDP at this time the data and the layer for header. 34 00:02:36,440 --> 00:02:40,520 ‫Here it's UDP header, which together form a segment or data. 35 00:02:40,520 --> 00:02:43,180 ‫Graham is passed down to layer three. 36 00:02:43,580 --> 00:02:44,510 ‫The network layer. 37 00:02:45,230 --> 00:02:47,690 ‫The network layer places it's layer three header. 38 00:02:47,740 --> 00:02:53,810 ‫Here, the IP header in front of the received segment and this group becomes a packet. 39 00:02:54,820 --> 00:03:01,510 ‫The layer three header contains important fields, such as the logical address, the IP address of both 40 00:03:01,510 --> 00:03:03,700 ‫the source and the destination device. 41 00:03:04,680 --> 00:03:11,430 ‫The newly formed packet is then passed down to layer two, the data link layer creates a new data unit 42 00:03:11,430 --> 00:03:16,890 ‫called the frame by adding the layer to frame header, which is the Ethernet header here. 43 00:03:18,020 --> 00:03:25,280 ‫Like layer three and addressing structure is also applied in the layer two header that is the Mac address. 44 00:03:26,030 --> 00:03:28,490 ‫The frame is then passed down to the physical layer. 45 00:03:29,530 --> 00:03:35,800 ‫Which converts the information into zero and one bits that are sent over the physical media using electrical 46 00:03:35,800 --> 00:03:38,530 ‫signals on a copper link, for instance. 47 00:03:39,490 --> 00:03:46,120 ‫Finally, the data is sent over the wire using a wide variety of methods such as Ethernet or token ring. 48 00:03:46,720 --> 00:03:52,750 ‫The headers are a specific form of control information, but allows the data to go through the network 49 00:03:52,750 --> 00:03:53,470 ‫properly. 50 00:03:53,860 --> 00:03:59,980 ‫Thus, the data at each layer is encapsulated in the information appropriate for this specific layer, 51 00:04:00,700 --> 00:04:03,520 ‫including addressing and error checking. 52 00:04:04,560 --> 00:04:10,800 ‫The overall size of the information increases as the data travels through the lower layers from layer 53 00:04:10,800 --> 00:04:17,760 ‫four to layer one, the destination device, which is the DNS server here, receives the data and this 54 00:04:17,760 --> 00:04:25,740 ‫additional information is analyzed and then is removed as the data passes through the higher layers 55 00:04:26,190 --> 00:04:30,810 ‫up to the application layer where the data is de capsule8 it. 56 00:04:31,860 --> 00:04:37,980 ‫The physical address, commonly the Mac address, which is located in a special field in the data link 57 00:04:37,980 --> 00:04:42,780 ‫layer header changes as the packet passes from one device to another. 58 00:04:43,380 --> 00:04:50,400 ‫For example, from the source PC to a switch to a router to another switch and finally to the destination 59 00:04:50,400 --> 00:04:50,910 ‫PC. 60 00:04:52,380 --> 00:04:58,260 ‫However, the original IP source and destination addresses do not change when transiting the network 61 00:04:58,920 --> 00:05:03,150 ‫because the packet is stripped of its layer three header only when it goes beyond a router. 62 00:05:04,440 --> 00:05:10,020 ‫When it stays within the same land, it only passes through switches, which capsule8 it at the layer 63 00:05:10,020 --> 00:05:11,970 ‫two header containing the Mac address. 64 00:05:13,310 --> 00:05:19,810 ‫As a result, the header changes as the packet is re encapsulated, as does the Mac address fields.