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What is Network and How Does it Work ? (Application Layer)

This is the 2nd lesson of this course. You are about to learn about Application layer. So, what is application layer and how does it work? The application layer is a layer in the Open Systems Interconnection (OSI) seven-layer model and in the TCP/IP protocol suite. It consists of protocols that focus on process-to-process communication across an IP network and provides a firm communication interface and end-user services.


Internet is the largest network in the world and it's popularity is primarily based on services and applications that it offers. People search the Internet because of the services like web pages, email, e-commerce, data transfer, P2P and many more. Complete idea of ​​the OSI model lies in the fact that the layers define functions. At the very top of the OSI model is an application layer, whose task is packing data in the message, define the appearance of the message and forwarding message to the lower, the transport layer.

Client/Server & P2P

Client-server architecture (client/server) is a network architecture in which each computer or process on the network is either a client or a server. Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers ). Let's take for example a website containing text, images and animations. On the one hand the user on his computer where the web browser sets the request to display the page. This request is forwarded through the network to the server's memory where the site is stored. Server then sends the reply to the user and the web page loads. The principle on which this example works is called Request/Replay.

Peer-to-peer (P2P) is a decentralized communications model in which each party has the same capabilities and either party can initiate a communication session. Unlike the client/server model, in which the client makes a service request and the server fulfills the request, the P2P network model allows each node to function as both a client and server. An example of P2P networks are Workgroups in the Microsoft Windows environment and various P2P programs that allow a file distribution on the internet.


The programs are the foundation of the user application layer and protocols are the network foundation. Protocols define the rules for communication between two application processes on remote PCs.
Protocols define next rules: 1.)Establish consistent rules for data exchange, 2.)Specify how data is structured within the message and define the types of messages that are sent to the network, 3.)Define protocol dialogue, ensuring that sent messages get the correct answers.

Review basic application layer protocols: Hyper Text Transfer Protocol (HTTP), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), Internet Message Access Protocol (IMAP), Domain Name System Protocol (DNS), Dynamic Host Configuration Protocol (DHCP), Server Message Block (SMB), Telnet Protocol.

HTTP is the most popular protocol today and one of the main reasons for the rapid increase in popularity of the Internet and computer networks in general. It is a protocol designed to transfer the contents of web pages. The basis of the contents of web pages is HTML. HTTP can transfer other content too and that is what it mainly does today. HTTP belongs to a client/server solution that defines the message format that is transmitted during their communication.

Generally speaking, the message header defines the type of message that is being sent, or define whether it is a request, response or something else. Also describes the content of the message and may specify additional features messages. The body of the message contains the data. Data can be the content of the website or some part of it (pictures, video, sound).

Methods are actions that have been implemented over the objects. The methods that are supported by HTTP are: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, and CONNECT.

FTP is used to transfer files between computers on a network. You can use FTP to exchange files between computer accounts, transfer files between an account and a desktop computer, or access online software archives. Keep in mind, however, that many FTP sites are heavily used and require several attempts before connecting.

FTP sessions work in passive or active modes. In active mode, after a client initiates a session via a command channel request, the server initiates a data connection back to the client and begins transferring data. In passive mode, the server instead uses the command channel to send the client the information it needs to open a data channel.

SMTP, POP & IMAP are the electronic mail protocols. In 1971, the former network ARPANet sent the first e-mail and launched something that will later change human communication. The content of the first message was "QWERTYUIOP". E-mail is a client/server solution, based application protocols. The client uses email program to connect to the server and receive or send mail.

DNS is like a phone book for the Internet. If you know a person’s name but don’t know their telephone number, you can simply look it up in a phone book. DNS provides this same service to the Internet.

When you visit in a browser, your computer uses DNS to retrieve the website’s IP address of x.x.x.x . Without DNS, you would only be able to visit a website by visiting its IP address directly, such as http://x.x.x.x.

DHCP is a client/server protocol that automatically provides an Internet Protocol (IP) host with its IP address and other related configuration information such as the subnet mask and default gateway.

Benefits of DHCP:
Reliable IP address configuration.
Reduced network administration.

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