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