"A computer network or data network is a telecommunications network that allows computers to exchange data. In computer networks, networked computing devices pass data to each other along data connections. The connections (network links) between nodes are established using either cable media or wireless media. The best-known computer network is the Internet."
Wikipedia
Wikipedia
The Open Systems Interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.
The model groups communication functions into seven logical layers. A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.
- The Application Layer (L7)
This is layer where the end users themselves interact with the network. Authentication services also run at L7. But encryption runs at the next layer down. The Application layer ensures that the remote communicate partner is availables that the needed communicate resources exist (a model for example) and that both ends of the communicate agree on procedures involving data integrity, privacy, and erro recovery.
When trying to decide if a protocol is an application layer protocol, just remember that protocol that require the end user to enter a request are application layer protocol. Firewall devices intended to keep network introder out, operate at L7. Some protocol and services that run at L7 include:
- HTTP - takes care of the communication between a web server and a web browser. HTTP is used for sending requests from a web client (a browser) to a web server, returning web content (web pages) from the server back to the client.
- SMTP - Simple Mail Transfer Protocol. SMTP is used for transmission of e-mails.
- Telnet - provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection.
- FTP - File Transfer Protocol . FTP takes care of transmission of files between computers.
- SNMP - Simple Network Management Protocol. SNMP is used for administration of computer networks.
- POP3 - Post Office Protocol version 3. The POP protocol is used by email programs (like Microsoft Outlook) to retrieve emails from an email server. If your email program uses POP, all your emails are downloaded to your email program (also called email client), each time it connects to your email server.
- The Presentation Layer (L6)
This layer answers on simple question: How should this data be presented? In additon to properly formatting data, encryption occurs at this layer. Have you ever opened a file in a work processing application and you got pages of unrecognizable characters? That's a Presentation layer issue. The application have not agree on how the data is to be presented. There are four primary tasks that Presentation layer is concerned with:
+ compatibility with th operating system
+ proper encapsulation of data for netwok transmission
+ data formatting (ASCII, binary)
+ data encryption, compression, and translation. You've probaly seen some of the file types that are used at the Presentation layer- JPEG, ASCII, GIF, MPEP, MIDI, EBC, DIC, and TIFF.
- The Session Layer (L5)
L5 is the "manager" of the two-way communicate between remote hosts. This is the layer that handles the creation, maintenace, and tear down of comunicate between those two hosts. The overall communication itself is referred to as a session.
Some session last jest long enough to send a unidirectional message, where other session will be of longer duration.
- The Transport Layer (L4)
The transport layer's purpose is to establish a logical end-to-end connection between to system, segment data receive from the upper layer of the OSI model and to make sure the data gets to the destination in the correct order and free of errors.
At the Transport layer, there are two methods for transporting data: connection-oriented, refering to TCP, and connectionless, refering to UDP. We'll take a much more detailed look at TCP and UDP later in this section.
- The Network Layer (L3)
It's at L3 of the OSI model that you and I as network admins begin to have a great deal of interaction with the network. IP run at this layer, and since routers operate here at L3. This layer is often called "the routing layer". In a nutshell, routing is a two-question process:
- What valid paths exist from the local router to a given destination?
- What is the best path to take to get there?
The Data-Link Layer (L2)
