0 1 00:00:00,570 --> 00:00:10,880 ...The top, or seventh, layer of the OSI model 1 2 00:00:10,940 --> 00:00:18,650 is the Application layer. The Application layer provides interfaces to the software that enable it 2 3 00:00:18,710 --> 00:00:21,070 to use network services. 3 4 00:00:21,140 --> 00:00:29,210 Some of the services provided by the Application layer include file transfer, electronic mail and web 4 5 00:00:29,210 --> 00:00:37,740 services. Applications that work at Layer 7 are the ones that users interact with directly. 5 6 00:00:37,790 --> 00:00:47,660 For example a web browser, Google Chrome, Firefox Safari and etc or the other applications such as Skype 6 7 00:00:48,020 --> 00:01:05,880 Outlook, Office - are examples of Layer 7 applications and HTTP, Telnet, FTP, SMTP, SNMP 7 8 00:01:06,060 --> 00:01:14,120 are common protocols used in Layer 7. in the client server module. 8 9 00:01:14,140 --> 00:01:23,770 In the application model, users can interact with client server model again. In 9 10 00:01:23,770 --> 00:01:37,290 this model as you know we have clients and we have a server. In this model clients are requesting 10 11 00:01:37,290 --> 00:01:47,090 some services from the server server provides the requested service for the clients. 11 12 00:01:48,460 --> 00:01:56,550 Another interaction method is Peer To Peer Model. In this model a 12 13 00:01:56,570 --> 00:02:03,830 user client can be a client or also can be a server at the same time. 13 14 00:02:04,000 --> 00:02:10,210 Most popular peer to peer applications are Torrent applications. 14 15 00:02:13,090 --> 00:02:20,050 In this section we are gonna talk about the common network protocols that clients use 15 16 00:02:20,050 --> 00:02:22,800 while they request services from servers. 16 17 00:02:22,930 --> 00:02:31,100 The first protocol we will talk about is HTTP which we use when we try to reach a web page 17 18 00:02:31,180 --> 00:02:36,850 mostly. HTTP is the underlying protocol used by the World Wide Web 18 19 00:02:36,850 --> 00:02:46,290 and this protocol defines how messages are formatted and transmitted, and what actions Web servers and browsers 19 20 00:02:46,300 --> 00:02:49,960 should take in response to various commands. 20 21 00:02:49,960 --> 00:02:58,540 For example, when you enter a URL in your browser, this actually sends an HTTP command to the 21 22 00:02:58,750 --> 00:03:11,440 Web server directing it to fetch and transmit the requested Web page. SMTP (Simple Mail Transfer Protocol) is a protocol used in sending 22 23 00:03:11,530 --> 00:03:15,310 and receiving e-mail and uses port 25. 23 24 00:03:15,550 --> 00:03:24,340 However, since it is limited in its ability to queue messages at the receiving end, it is usually used 24 25 00:03:24,340 --> 00:03:34,030 with one of two other protocols, POP3 or IMAP, that let the user save messages in a server 25 26 00:03:34,030 --> 00:03:38,570 mailbox and download them periodically from the server 26 27 00:03:38,850 --> 00:03:46,420 Pop protocol is Used by local e-mail clients to retrieve e-mail from a remote server. 27 28 00:03:46,660 --> 00:03:55,540 POP works with Simple Mail Transfer Protocol (SMTP) for end-to-end email communication, where 28 29 00:03:55,540 --> 00:03:57,420 POP pulls messages 29 30 00:03:57,460 --> 00:04:07,750 IMAP is als Used by local e-mail clients to retrieve e-mail 30 31 00:04:07,750 --> 00:04:09,650 from a remote server. 31 32 00:04:09,850 --> 00:04:19,030 IMAP can be contrasted with POP With POP3, mail is saved for the end user in a single mailbox 32 33 00:04:19,060 --> 00:04:27,070 on the server and moved to the end user's device when the mail client opens. While POP3 33 34 00:04:27,180 --> 00:04:31,050 can be thought of as a "store-and-forward" service, 34 35 00:04:31,100 --> 00:04:39,880 IMAP can be thought of as a remote file server. Original copies are stored in mail server until 35 36 00:04:39,880 --> 00:04:41,850 it is deleted 36 37 00:04:42,400 --> 00:04:52,450 And let's go ahead with the DNS; domain name system .DNS is used to convert a computer's hostname into 37 38 00:04:52,450 --> 00:04:55,210 an IP address on the Internet. 38 39 00:04:55,300 --> 00:05:03,970 For example, when you type www.Google.com , there is a DNS server serving on internet 39 40 00:05:04,000 --> 00:05:11,570 or in your local network is responding you that www.google.com's 40 41 00:05:11,620 --> 00:05:14,430 IP address is this one. 41 42 00:05:14,440 --> 00:05:19,780 So what you exactly doing is going to this IP address 42 43 00:05:22,400 --> 00:05:25,660 DNS uses a hierarchy to manage 43 44 00:05:25,670 --> 00:05:33,560 its distributed database system. The DNS hierarchy, also called the domain name space, 44 45 00:05:33,560 --> 00:05:42,200 is an inverted tree structure, much like a Directory. The DNS tree has a single domain at the top of the 45 46 00:05:42,200 --> 00:05:51,770 structure called the root domain. A period or dot (.) is the designation for the root domain. 46 47 00:05:51,830 --> 00:05:55,440 Below the root domain are the top-level domains that divide 47 48 00:05:55,460 --> 00:06:04,940 the DNS hierarchy into segments like .edu for educational organizations, .gov for government 48 49 00:06:04,970 --> 00:06:14,690 agencies, .com for commercial organizations etc. The next level in the DNS hierarchy is 49 50 00:06:14,690 --> 00:06:17,270 the second level domains. 50 51 00:06:17,330 --> 00:06:21,900 This is the domain that is directly below the tld. 51 52 00:06:21,950 --> 00:06:24,950 This is the main part of the domain name. 52 53 00:06:24,950 --> 00:06:34,580 It can vary according to the buyer. For example : contoso.com The sub-domain can be defined as the domain 53 54 00:06:34,610 --> 00:06:37,070 that is a part of the main domain. 54 55 00:06:37,070 --> 00:06:48,270 For example : example.contoso.com You can configure the IP addresses of the network devices in 55 56 00:06:48,360 --> 00:06:49,770 two ways. 56 57 00:06:49,830 --> 00:06:53,280 The first way is static configuration , 57 58 00:06:53,400 --> 00:06:59,400 the second one is using DHCP. By using DHCP 58 59 00:06:59,580 --> 00:07:08,040 You Can assign IP address, subnet mask, gateway, DNS and domain name to a network device 59 60 00:07:08,280 --> 00:07:09,360 dynamically 60 61 00:07:09,360 --> 00:07:17,010 If you are managing a small network, you can statically assign IP address configurations to your devices , 61 62 00:07:17,010 --> 00:07:23,600 but for big corporate networks DHCP is almost mandotary. 62 63 00:07:23,670 --> 00:07:25,630 There is a process 63 64 00:07:25,650 --> 00:07:36,480 While a PC is demanding an IP address from a DHCP server called as the Dora process. the first step of 64 65 00:07:36,480 --> 00:07:42,230 the process is the DHCP discover message , in this message 65 66 00:07:42,240 --> 00:07:44,390 This PC is saying help, 66 67 00:07:44,430 --> 00:07:47,100 I don't have an IP address for example. 67 68 00:07:47,100 --> 00:07:52,920 This is a broadcast message and searching for an IP address. 68 69 00:07:53,030 --> 00:08:02,350 Then server gets this message and responds with the DHCP offer unicast message and says that 69 70 00:08:02,370 --> 00:08:04,140 How about this address? 70 71 00:08:04,170 --> 00:08:06,120 I have an IP address for you. 71 72 00:08:06,120 --> 00:08:07,890 Do you want it or not? 72 73 00:08:08,110 --> 00:08:12,020 OK the client takes this message and say that 73 74 00:08:12,030 --> 00:08:16,450 Can't I just use the address I have before for example or. 74 75 00:08:16,500 --> 00:08:22,570 That can also accept the IP address that the server offers. 75 76 00:08:22,770 --> 00:08:31,540 And this is the message saying that for example can't I just use the address I had before or it's ok I'm 76 77 00:08:31,620 --> 00:08:33,080 accepting your offer. 77 78 00:08:33,090 --> 00:08:45,330 Is a DHCP request message and sent broadcast again then last step is the acknowledgment and 78 79 00:08:45,390 --> 00:08:53,710 server responds back with an acknowledgement message again and says it is ok no problem 79 80 00:08:53,880 --> 00:08:58,200 I'm assigning the IP that you want from me and 80 81 00:08:58,260 --> 00:08:59,190 OK. 81 82 00:08:59,390 --> 00:09:02,970 Awesome I'm somebody now 82 83 00:09:05,690 --> 00:09:08,840 another application layer protocol we are going to focus on 83 84 00:09:08,840 --> 00:09:17,090 lastly is the file transfer protocol. File Transfer Protocol is Used to transfer computer files between a 84 85 00:09:17,090 --> 00:09:19,910 client and server on a network. 85 86 00:09:19,970 --> 00:09:24,560 This protocol uses port number 21.