1 1 00:00:00,300 --> 00:00:02,620 Data Transfer Over Networks. 2 2 00:00:02,620 --> 00:00:04,700 So, when we transfer data over our networks, 3 3 00:00:04,700 --> 00:00:08,100 we have to find a way to tell the data where it's going to go. 4 4 00:00:08,100 --> 00:00:11,320 Now, we use IP addresses to get data to the right system. 5 5 00:00:11,320 --> 00:00:12,920 But how does it know what application 6 6 00:00:12,920 --> 00:00:15,310 is listening on that particular system? 7 7 00:00:15,310 --> 00:00:17,490 Well, that's where a port comes in. 8 8 00:00:17,490 --> 00:00:19,040 I like to think about it this way, 9 9 00:00:19,040 --> 00:00:20,580 if I give you my street address, 10 10 00:00:20,580 --> 00:00:22,260 that's like my IP address. 11 11 00:00:22,260 --> 00:00:25,050 You can find my exact house within my neighborhood. 12 12 00:00:25,050 --> 00:00:27,210 But, how are you going to find my bedroom, 13 13 00:00:27,210 --> 00:00:29,260 my home office, or my living room. 14 14 00:00:29,260 --> 00:00:31,750 Each of those rooms is going to serve a different function, 15 15 00:00:31,750 --> 00:00:34,310 like sleeping, working, or relaxing. 16 16 00:00:34,310 --> 00:00:36,850 Well, this is where a port is going to come in. 17 17 00:00:36,850 --> 00:00:39,120 A Port is a logical opening in a computer 18 18 00:00:39,120 --> 00:00:41,230 that represents a service or application 19 19 00:00:41,230 --> 00:00:43,490 that's listening and waiting for traffic. 20 20 00:00:43,490 --> 00:00:45,250 So, in my house example, 21 21 00:00:45,250 --> 00:00:48,350 maybe I gave you my address as 123 Main Street, 22 22 00:00:48,350 --> 00:00:49,730 room number two. 23 23 00:00:49,730 --> 00:00:52,770 This tells you to go to the house and enter the second room, 24 24 00:00:52,770 --> 00:00:56,030 which is my home office where I might be working at. 25 25 00:00:56,030 --> 00:00:58,690 Now, in computer networks, our ports are going to be numbered 26 26 00:00:58,690 --> 00:01:02,910 from zero all the way up to 65,535. 27 27 00:01:02,910 --> 00:01:04,700 That is a lot of ports. 28 28 00:01:04,700 --> 00:01:07,370 So, even though you only have one IP on your machine, 29 29 00:01:07,370 --> 00:01:10,370 you can have over 65,000 different openings 30 30 00:01:10,370 --> 00:01:12,170 for you to be able to run different services 31 31 00:01:12,170 --> 00:01:13,870 and applications on. 32 32 00:01:13,870 --> 00:01:16,530 Now, that large number of ports is actually broken up 33 33 00:01:16,530 --> 00:01:18,320 into two distinct groups. 34 34 00:01:18,320 --> 00:01:21,400 The first is called the Well-known and Reserved Ports. 35 35 00:01:21,400 --> 00:01:22,710 These are the ports that are numbered 36 36 00:01:22,710 --> 00:01:25,360 from zero through a 1023. 37 37 00:01:25,360 --> 00:01:27,710 These include things like the File Transfer Protocol 38 38 00:01:27,710 --> 00:01:30,620 on port 21, Web Browsing on port 80, 39 39 00:01:30,620 --> 00:01:33,480 or the Simple Mail Transfer Protocol on port 25, 40 40 00:01:33,480 --> 00:01:35,150 among other things. 41 41 00:01:35,150 --> 00:01:36,730 Ephemeral Ports, on the other hand, 42 42 00:01:36,730 --> 00:01:38,650 are short-lived temporary ports. 43 43 00:01:38,650 --> 00:01:41,060 And these are open for just a small period of time 44 44 00:01:41,060 --> 00:01:43,320 from a predefined range of ports. 45 45 00:01:43,320 --> 00:01:46,390 These ports can exist from Port 1024, 46 46 00:01:46,390 --> 00:01:50,100 all the way up to 65,535. 47 47 00:01:50,100 --> 00:01:52,450 For example, as I'm recording this lesson, 48 48 00:01:52,450 --> 00:01:55,720 I record the audio on a device which acts a file server, 49 49 00:01:55,720 --> 00:01:57,400 and it's going to pick a random port 50 50 00:01:57,400 --> 00:01:59,730 somewhere around port 60,000. 51 51 00:01:59,730 --> 00:02:02,430 Now, when I want to download the audio file from the device, 52 52 00:02:02,430 --> 00:02:04,440 I'm going to use a web browser on my computer 53 53 00:02:04,440 --> 00:02:07,280 to access it and download that audio file. 54 54 00:02:07,280 --> 00:02:09,930 When I do that, what happened is that I just opened up 55 55 00:02:09,930 --> 00:02:12,600 a port on the device for as long as it's needed 56 56 00:02:12,600 --> 00:02:14,280 in order to download that file 57 57 00:02:14,280 --> 00:02:16,160 and then the port is going to close up again, 58 58 00:02:16,160 --> 00:02:17,800 just that quickly. 59 59 00:02:17,800 --> 00:02:20,510 Now, data transfer occurs using these IP addresses 60 60 00:02:20,510 --> 00:02:21,370 in these ports. 61 61 00:02:21,370 --> 00:02:24,510 So, what does all this look like inside the real world? 62 62 00:02:24,510 --> 00:02:27,170 Well, as you can see here, I have a Client on the left 63 63 00:02:27,170 --> 00:02:28,730 and a Website on the right. 64 64 00:02:28,730 --> 00:02:33,730 The Client is at the address 192.168.1.24, 65 65 00:02:33,800 --> 00:02:38,800 and the Website is at 64.82.46.21. 66 66 00:02:39,190 --> 00:02:41,980 So, for the client to make a request to this website, 67 67 00:02:41,980 --> 00:02:43,940 it's going to come from the source IP, 68 68 00:02:43,940 --> 00:02:46,380 that 192 address, and a port, 69 69 00:02:46,380 --> 00:02:48,370 just some random high port that it's going to pick 70 70 00:02:48,370 --> 00:02:50,090 from the Ephemeral port range. 71 71 00:02:50,090 --> 00:02:54,750 Now, in this case, it shows port number 49,163, 72 72 00:02:54,750 --> 00:02:57,030 but it could have been any Ephemeral port. 73 73 00:02:57,030 --> 00:02:58,970 Now, we're going to take that data 74 74 00:02:58,970 --> 00:03:01,680 and we're going to send it to the destination of the website, 75 75 00:03:01,680 --> 00:03:03,710 which in this case is going to be the IP address 76 76 00:03:03,710 --> 00:03:07,140 of 64. something that you see here on the web server. 77 77 00:03:07,140 --> 00:03:09,700 This is going to act as our destination IP. 78 78 00:03:09,700 --> 00:03:12,330 And what port do you think it's going to go to? 79 79 00:03:12,330 --> 00:03:14,760 Well, it's going to use the default port for websites, 80 80 00:03:14,760 --> 00:03:16,140 which is Port 80. 81 81 00:03:16,140 --> 00:03:17,810 Now, the website, on the other hand 82 82 00:03:17,810 --> 00:03:19,310 is going to reverse this process. 83 83 00:03:19,310 --> 00:03:21,170 And it's going to go from its source IP 84 84 00:03:21,170 --> 00:03:23,580 and its own port 80 back to the client 85 85 00:03:23,580 --> 00:03:28,310 on its random high port, which was port 49,163. 86 86 00:03:28,310 --> 00:03:30,560 So, you can see how this communication happens 87 87 00:03:30,560 --> 00:03:31,940 to establish this session. 88 88 00:03:31,940 --> 00:03:34,080 And then, they can communicate back and forth 89 89 00:03:34,080 --> 00:03:37,750 between the two servers and the client over these two ports. 90 90 00:03:37,750 --> 00:03:39,900 Whenever we're done transmitting this website, 91 91 00:03:39,900 --> 00:03:42,520 that random high port that was opened on your client, 92 92 00:03:42,520 --> 00:03:44,940 it's going to get closed and another random high port 93 93 00:03:44,940 --> 00:03:47,520 will be opened the next time you need to communicate. 94 94 00:03:47,520 --> 00:03:49,410 But back on the web server, 95 95 00:03:49,410 --> 00:03:52,560 port 80 is going to sit there and always remain open 96 96 00:03:52,560 --> 00:03:54,700 ready to serve requests from anybody 97 97 00:03:54,700 --> 00:03:57,140 who wants to communicate with that website. 98 98 00:03:57,140 --> 00:03:59,960 So, when we send the information across the network, 99 99 00:03:59,960 --> 00:04:02,730 we're doing this as an IPv4 packet. 100 100 00:04:02,730 --> 00:04:06,200 Now, IP version four packets consist of a source address, 101 101 00:04:06,200 --> 00:04:08,010 that's the IP address of the sender, 102 102 00:04:08,010 --> 00:04:11,100 a destination address, the IP address of the receiver, 103 103 00:04:11,100 --> 00:04:14,340 the IP flags, which is going to allow the packet fragmentation 104 104 00:04:14,340 --> 00:04:15,740 other special things to occur, 105 105 00:04:15,740 --> 00:04:18,210 and the protocol, which is whether we're going to communicate 106 106 00:04:18,210 --> 00:04:20,730 using TCP or UDP. 107 107 00:04:20,730 --> 00:04:23,450 All of those things are going to make up this packet header 108 108 00:04:23,450 --> 00:04:25,700 and you can see it shown here on the screen. 109 109 00:04:25,700 --> 00:04:27,730 Now, many times students asked me 110 110 00:04:27,730 --> 00:04:30,510 if they have to memorize this packet header for the exam. 111 111 00:04:30,510 --> 00:04:33,660 Well, the answer is no, no, no, no, no, no. 112 112 00:04:33,660 --> 00:04:35,140 You don't have to worry about it, 113 113 00:04:35,140 --> 00:04:37,500 because no one is going to ask you to draw out 114 114 00:04:37,500 --> 00:04:39,750 or diagram a packet for the exam. 115 115 00:04:39,750 --> 00:04:42,540 That said, I do want you to understand that your header 116 116 00:04:42,540 --> 00:04:45,010 contains a couple of key pieces of information. 117 117 00:04:45,010 --> 00:04:47,290 Things like your source and destination IP, 118 118 00:04:47,290 --> 00:04:49,220 as well as the ports and the protocols 119 119 00:04:49,220 --> 00:04:50,550 and things of that nature. 120 120 00:04:50,550 --> 00:04:53,680 This tells your packet where it needs to get routed to. 121 121 00:04:53,680 --> 00:04:56,810 Now, back when we talked about TCP and UDP, 122 122 00:04:56,810 --> 00:05:00,330 we talked about the fact that TCP had a lot more overhead. 123 123 00:05:00,330 --> 00:05:01,940 But, what does that really look like 124 124 00:05:01,940 --> 00:05:03,480 inside of these packets? 125 125 00:05:03,480 --> 00:05:05,610 Well, that's what I have here on the screen. 126 126 00:05:05,610 --> 00:05:08,000 On the top, you can see the TCP header, 127 127 00:05:08,000 --> 00:05:09,600 which is 20 bytes in length. 128 128 00:05:09,600 --> 00:05:11,510 And you can see some things that it contains, 129 129 00:05:11,510 --> 00:05:13,900 like the Source Port and the Destination Port, 130 130 00:05:13,900 --> 00:05:16,070 the Sequence Number, the Acknowledgement Number, 131 131 00:05:16,070 --> 00:05:18,030 and then a bunch of other flags. 132 132 00:05:18,030 --> 00:05:20,130 All of that with the Checksum and the Pointers, 133 133 00:05:20,130 --> 00:05:22,130 it all adds this overall length, 134 134 00:05:22,130 --> 00:05:24,760 making it a pretty bulky protocol. 135 135 00:05:24,760 --> 00:05:28,350 Now, we can compare this TCP header with a UDP header 136 136 00:05:28,350 --> 00:05:29,930 that's shown here on the bottom. 137 137 00:05:29,930 --> 00:05:31,870 It only has four pieces of information. 138 138 00:05:31,870 --> 00:05:34,370 It has a Source Port, a Destination Port, 139 139 00:05:34,370 --> 00:05:36,510 the length of the packet and the Checksum. 140 140 00:05:36,510 --> 00:05:39,010 That's it, and it's only eight bytes long. 141 141 00:05:39,010 --> 00:05:40,950 So, you can see there's a lot less data there 142 142 00:05:40,950 --> 00:05:43,380 in that UDP header because there's less 143 143 00:05:43,380 --> 00:05:44,410 that needs to be contained. 144 144 00:05:44,410 --> 00:05:46,500 And so, it allows us to have a faster, 145 145 00:05:46,500 --> 00:05:48,090 more efficient protocol. 146 146 00:05:48,090 --> 00:05:50,040 Again, you don't need to memorize these headers 147 147 00:05:50,040 --> 00:05:50,873 for the exam. 148 148 00:05:50,873 --> 00:05:53,240 But you should remember that TCP is bulkier 149 149 00:05:53,240 --> 00:05:54,660 with its 20-byte header 150 150 00:05:54,660 --> 00:05:57,010 than UDP is with its 8-byte header. 151 151 00:05:57,010 --> 00:06:00,000 Because of this additional overhead in the TCP protocol, 152 152 00:06:00,000 --> 00:06:02,373 UDP is considered much faster.