WEBVTT 00:00:01.040 --> 00:00:03.910 We've taken a look at the seven layers of the Open 00:00:03.910 --> 00:00:06.790 Systems Interconnection model, but as we said, 00:00:06.790 --> 00:00:09.870 we don't really use it, except as a reference. 00:00:09.870 --> 00:00:13.480 Instead, for the most part, we use TCP/IP. 00:00:13.480 --> 00:00:18.320 TCP/IP actually was developed before OSI, 00:00:18.320 --> 00:00:22.510 but some of the problems with TCP/IP are that we can't even 00:00:22.510 --> 00:00:26.280 agree on what the various layers should be called or even 00:00:26.280 --> 00:00:28.640 sometimes how many layers there are. 00:00:28.640 --> 00:00:34.060 Many people argue there should be three, four, or five layers to TCP/IP. 00:00:34.060 --> 00:00:36.050 It doesn't matter. 00:00:36.370 --> 00:00:39.610 The whole point is that's why we often use OSI because 00:00:39.610 --> 00:00:41.610 it is more defined and structured. 00:00:42.790 --> 00:00:46.480 We used the four‑layer model here because that came out of the 00:00:46.480 --> 00:00:50.080 original development of the DARPA and DARPANET projects, 00:00:50.080 --> 00:00:55.740 but that included then taking the top three layers of the OSI model, 00:00:55.790 --> 00:00:58.090 application, presentation, and session, 00:00:58.090 --> 00:01:03.700 and combining them into the application layer and taking the bottom two layers, 00:01:03.710 --> 00:01:08.740 data link and physical, and combining them into the network access layer. 00:01:10.120 --> 00:01:13.800 One of the things that makes the internet work for you 00:01:13.800 --> 00:01:16.100 and me is the Domain Name System. 00:01:16.100 --> 00:01:20.300 You see, you and I like to talk in regular, 00:01:20.300 --> 00:01:26.370 in my case, English, but the internet, routers for example, 00:01:26.370 --> 00:01:30.290 are based on binary values, they don't understand English. 00:01:30.600 --> 00:01:38.950 So, we use a natural name that needs to be then translated into a binary value, 00:01:38.950 --> 00:01:41.880 and that is done by DNS. 00:01:41.880 --> 00:01:45.890 DNS takes that natural name that we type in, 00:01:45.890 --> 00:01:47.050 pluralsight.com, 00:01:47.050 --> 00:01:53.870 and converts it into the IP address of pluralsight.com so routers know how 00:01:53.870 --> 00:01:57.250 to route that traffic towards its correct destination. 00:01:57.980 --> 00:02:02.790 For example, if you typed in www.apple.com, 00:02:03.230 --> 00:02:10.930 it would be then translated into the IP address of 17.253.144.10. 00:02:12.030 --> 00:02:15.910 The problem, of course, is that if DNS goes down, 00:02:15.910 --> 00:02:20.040 then really, most users won't be able to get to a website. 00:02:20.040 --> 00:02:25.080 If you knew the IP address, 17.253 for example, 00:02:25.200 --> 00:02:29.250 you could get to Apple without ever typing in apple.com. 00:02:29.520 --> 00:02:33.140 But since very few of us know what those IP addresses are, 00:02:33.140 --> 00:02:39.850 we rely on DNS as really a telephone book that has conversion that says, 00:02:39.850 --> 00:02:42.960 this is the number that belongs to this name. 00:02:44.750 --> 00:02:48.910 One of the things that makes the internet work is the use of ports. 00:02:48.910 --> 00:02:51.740 Ports, we could call them doorways. 00:02:52.000 --> 00:02:55.440 They're used to control traffic and there's a number of 00:02:55.440 --> 00:02:57.930 common ports we use all the time. 00:02:58.420 --> 00:03:01.340 For example, if I'm going to transfer files, 00:03:01.340 --> 00:03:05.460 I'll very often use a protocol known as File Transfer Protocol 00:03:05.660 --> 00:03:08.960 that runs on the doorway marked 20 and 21. 00:03:09.830 --> 00:03:13.500 If I'm going to transmit traffic using a secure shell, 00:03:13.700 --> 00:03:17.310 that would be port number or door number 22. 00:03:17.310 --> 00:03:22.830 TELNET, terminal emulation network, 23; email, 00:03:22.830 --> 00:03:26.950 Simple Mail Transfer Protocol, runs on port 25. 00:03:27.470 --> 00:03:32.180 The DNS, which we just looked at, requires port 53 to be open, 00:03:32.180 --> 00:03:36.030 and that's where DNS traffic and resolvers would come in. 00:03:36.150 --> 00:03:38.730 So if I typed in apple.com, 00:03:38.730 --> 00:03:43.060 the DNS system could communicate over that door to be 00:03:43.060 --> 00:03:45.490 able to tell us what the IP address was. 00:03:46.250 --> 00:03:49.640 And the most common one we use when we're surfing the internet, 00:03:49.640 --> 00:03:55.250 of course, is port 80, which is used for hypertext transfer protocol traffic. 00:03:56.170 --> 00:04:00.960 We also have Post Office Protocol 3, which runs on port number 110, 00:04:00.960 --> 00:04:03.770 and when we're sending encrypted traffic, 00:04:03.770 --> 00:04:07.830 say to a bank, we use Transport Layer Security, 00:04:07.830 --> 00:04:13.920 TLS, which was an adaptation and improvement on the former SSL, 00:04:13.920 --> 00:04:18.050 Secure Sockets Layer, which runs on port 443. 00:04:18.050 --> 00:04:20.279 Where are these doors located? 00:04:20.279 --> 00:04:21.490 On a firewall. 00:04:22.040 --> 00:04:26.750 The firewall has a number of doors that are open to certain traffic, 00:04:26.750 --> 00:04:32.790 but will block traffic to destinations and types of traffic that it says, 00:04:32.790 --> 00:04:37.730 no, I'm not going to allow any traffic through a different port. 00:04:39.780 --> 00:04:41.550 The Key Points Review. 00:04:41.550 --> 00:04:45.540 We've covered a tremendous amount of network terminology 00:04:45.540 --> 00:04:47.840 and theory here in just a few minutes, 00:04:47.840 --> 00:04:51.390 but it's good to go over this again and make sure I've understood 00:04:51.390 --> 00:04:53.680 it if I'm not really familiar with networks, 00:04:53.680 --> 00:04:57.350 because I can't really secure a network until I have some 00:04:57.350 --> 00:04:59.650 understanding of how a network works. 00:05:00.470 --> 00:05:05.200 We saw how OSI is an excellent reference model that will help us 00:05:05.210 --> 00:05:08.980 understand the process of network‑based communications. 00:05:09.720 --> 00:05:16.510 TCP/IP is the basis used by many devices today and systems to communicate, 00:05:16.510 --> 00:05:19.680 even including things like our cars, for example, 00:05:19.730 --> 00:05:25.850 many of them run almost entirely on TCP/IP communications today.