Networking - The OSI Seven-Layer Model

A network is any two or more entities -- be they people, organizations, machines, or whatever -- that:

Need to share a resource (in computer geekspeak referred to as services),
Have a pathway for contacting one another (referred to as transmission media), and
Have rules for using their common pathway to communicate about the sharing of resources (referred to as protocols).

Before a network can be effective, participants must have clearly established these three things.

In computer networking, fortunately, the services, transmission media, and protocols have long since been codified so that the uninitiated can quickly master the concepts.

Since 1977, the International Standards Organization (ISO) has promulgated the Open Systems Interconnection (OSI) Reference Model. The OSI seven-layer model, as it has come to be known, consists of (get this!) seven layers:

Application (Layer 7)
Presentation (Layer 6)
Session (Layer 5)
Transport (Layer 4)
Network (Layer 3)
Data Link (Layer 2)
Physical (Layer 1)

The seven layers collectively are often referred to as a protocol stack, although in actuality only the transport and higher layers deal directly with protocols, per se.

Each layer is responsible for specific functions of network communications. It is important to note, however, that the model is theoretical; actual network operating systems (NOSs) handle the practical implementation of each of these theoretical functions in different ways. Nevertheless, the model does help clarify the relationships and define the interactions the multiple protocol stacks of the various NOSs have to and with one another.

In networking, each layer in a computer's protocol stack communicates with the corresponding layers in other computers' stacks. This works both horizontally and vertically:

As data is passed from the Transport layer in one computer, it is received and interpreted by the Transport layer in other computers. However,
To get to the corresponding layers in other computers, the data must pass down through the other layers of the originating computer and up through the lower layers of the receiving computers. As the data is handed down to lower layers, each layer adds instructions and other information (called a header) to the beginning of the data. Then, as the data arrives at each layer of the receiving computer, the corresponding headers are stripped off and any instructions are carried out before the data is handed up to the next layer.

Let's get into the meat of the layers, starting at the bottom with layer 1, the Physical Layer.

Nah, I want to skip ahead:

Introduction
Physical (Layer 1)
Data Link (Layer 2)
Network (Layer 3)
Transport (Layer 4)
Session (Layer 5)
Presentation (Layer 6)
Application (Layer 7)
Conclusion