What is OSI model? OSI(open system interconection) is a conceptual framework that describe network communication as a series of seven layer. In the model, each layer is responsible for a different part of the process that takes place when two computers on a network establish a connection and move data between them.if you want to became such kind of network proffesional(sound cool huh), you must understand bout this kinda thing.even a good hacker must understand this all first.for hacker and anti-hacker this is like their guide about their playing ground in the net.interesting? read it!

Okay lets go to the point. Those seven layer is like below(from the seventh to first).

1.Application layer

Provides the interface and services that support user applications and provides general access to the network. Because it is at the top of the OSI model, it does not provide services to layers above it (because there are no
layers above it), but it does provide network services related to user applications such as message handling, file transfer, and database queries.
Example : FTP service for file transfer file, HTTP service for web,
SMTP/IMAP/POP3 service for mail.

2.Presentation layer

You can think this layer as a translator layer of the OSI model. It is responsible for data conversion and encryption.

3.Session layer

Establishes and maintains the communication link between the sending and receiving nodes. There are 3 kind of communication session setup between the network node, simplex, half-duplex, full-duplex. Simplex transmits data in only one direction (such as your thermostat connecting to your furnace). Half-duplex allows for communication in two directions but only allows for the transmission of data in one direction at a time. Full-duplex allows communication in both directions at the same time.

4.Transport layer

Responsible for end-to-end data transmission, flow control, error checking, and recovery. The Transport layer also provides two different types of data delivery methods: connection-oriented and connectionless. Connection-oriented transport uses a system of acknowledgements to ensure data delivery and defines a static route on the network so that the packets are delivered along the same route during the session. This is considered a reliable connection. you can think of it as telephone. You call your friend, and he/she answer the telephone. When you both agree to finish your conversation, you both will close the conversation. Example: TCP protocol. But different with the connectionless method. It just send the destination with data, without know they receive it or not. It’s just like a post mail placed in your home mail box. Example: UDP protocol.

5.Network layer

Provides the logical addressing system used to route data on the network. This logical address is your IP address. it contain 4 segment separated by dot, which each segment is a number between 0 – 255. Example 202.46.129.10.
another protocol that work in this layer is RIP(Routing Information Protocol) and ARP(Address resolution Protocol).

6.Data Link

Responsible for the framing of data packets and the movement of the data across the physical link. It is according to your network architecture. An ethernet network will framed by ethernet frame, and ring network by token ring
frame.

7.Physical

Manages the process of sending and receiving bits over the physical network media (the wire and other physical devices). From this checkpoints you already know about those layer, but what the connection between the data that
transfered with this layer? confuse huh? let see… this is simple explaination :

  • Sender

Data(your email message as an example) –> aplication layer(get a header of this layer) –> presentasion layer (get a header again from this layer) –> Session layer(get a header of this layer) –> Transport layer (get a header of this layer) –> Network layer(get a header of this layer) –> Data link layer (get a header of this layer) –> Physical layer –> sent on physical layer there is no header embeded. and your data here is in bit stream form.

  • Receiver

just like sender but data flow from physical layer –> data link layer –> network layer –> transport layer –> session layer –> presentation layer –> aplication layer.

in each layer, the header was opened, and after reach the aplication layer, your data will be back to the form like what it was typed by the sender.

reference : backtrackbox.com