All language subtitles for 12. Application Layer

...The top, or seventh, layer of the OSI model
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is the Application layer. The Application layer provides interfaces to the software that enable it
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to use network services.
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Some of the services provided by the Application layer include file transfer, electronic mail and web
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services. Applications that work at Layer 7 are the ones that users interact with directly.
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For example a web browser, Google Chrome, Firefox Safari and etc or the other applications such as Skype
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Outlook, Office - are examples of Layer 7 applications and HTTP, Telnet, FTP, SMTP, SNMP
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are common protocols used in Layer 7.
in the client server module.
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In the application model, users can interact with client server model again. In
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this model as you know we have clients and we have a server. In this model clients are requesting
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some services from the server server provides the requested service for the clients.
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Another interaction method is Peer To Peer Model. In this model a
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user client can be a client or also can be a server at the same time.
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Most popular peer to peer applications are Torrent applications.
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In this section we are gonna talk about the common network protocols that clients use
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while they request services from servers.
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The first protocol we will talk about is HTTP which we use when we try to reach a web page
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mostly. HTTP is the underlying protocol used by the World Wide Web
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and this protocol defines how messages are formatted and transmitted, and what actions Web servers and browsers
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should take in response to various commands.
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For example, when you enter a URL in your browser, this actually sends an HTTP command to the
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Web server directing it to fetch and transmit the requested Web page. SMTP (Simple Mail Transfer Protocol) is a protocol used in sending
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and receiving e-mail and uses port 25.
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However, since it is limited in its ability to queue messages at the receiving end, it is usually used
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with one of two other protocols, POP3 or IMAP, that let the user save messages in a server
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mailbox and download them periodically from the server
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Pop protocol is Used by local e-mail clients to retrieve e-mail from a remote server.
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POP works with Simple Mail Transfer Protocol (SMTP) for end-to-end email communication, where
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POP pulls messages
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IMAP is als Used by local e-mail clients to retrieve e-mail
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from a remote server.
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IMAP can be contrasted with POP With POP3, mail is saved for the end user in a single mailbox
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on the server and moved to the end user's device when the mail client opens. While POP3
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can be thought of as a "store-and-forward" service,
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IMAP can be thought of as a remote file server. Original copies are stored in mail server until
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it is deleted
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And let's go ahead with the DNS; domain name system .DNS is used to convert a computer's hostname into
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an IP address on the Internet.
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For example, when you type www.Google.com , there is a DNS server serving on internet
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or in your local network is responding you that www.google.com's
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IP address is this one.
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So what you exactly doing is going to this IP address
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DNS uses a hierarchy to manage
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its distributed database system. The DNS hierarchy, also called the domain name space,
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is an inverted tree structure, much like a Directory.
The DNS tree has a single domain at the top of the
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structure called the root domain. A period or dot (.) is the designation for the root domain.
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Below the root domain are the top-level domains that divide
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the DNS hierarchy into segments like .edu for educational organizations, .gov for government
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agencies, .com for commercial organizations etc.
The next level in the DNS hierarchy is
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the second level domains.
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This is the domain that is directly below the tld.
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This is the main part of the domain name.
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It can vary according to the buyer. For example : contoso.com
The sub-domain can be defined as the domain
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that is a part of the main domain.
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For example : example.contoso.com
You can configure the IP addresses of the network devices in
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two ways.
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The first way is static configuration ,
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the second one is using DHCP. By using DHCP
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You Can assign IP address, subnet mask, gateway, DNS and domain name to a network device
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dynamically
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If you are managing a small network, you can statically assign IP address configurations to your devices ,
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but for big corporate networks DHCP is almost mandotary.
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There is a process
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While a PC is demanding an IP address from a DHCP server called as the Dora process. the first step of
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the process is the DHCP discover message , in this message
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This PC is saying help,
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I don't have an IP address for example.
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This is a broadcast message and searching for an IP address.
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Then server gets this message and responds with the DHCP offer unicast message and says that
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How about this address?
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I have an IP address for you.
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Do you want it or not?
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OK the client takes this message and say that
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Can't I just use the address I have before for example or.
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That can also accept the IP address that the server offers.
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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
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accepting your offer.
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Is a DHCP request message and sent broadcast again then last step is the acknowledgment and
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server responds back with an acknowledgement message again and says it is ok no problem
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I'm assigning the IP that you want from me and
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OK.
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Awesome I'm somebody now
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another application layer protocol we are going to focus on
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lastly is the file transfer protocol. File Transfer Protocol is Used to transfer computer files between a
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client and server on a network.
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This protocol uses port number 21.