L2 network topologyBefore you dive into what you may want to document and include in a network diagram, you’ll probably want to have a basic understand of what, exactly, the OSI model is.

The Open Systems Interconnection Model (OSI) is, in a nutshell, a theoretical or reference model which defines how the different applications in a network communicate with one another. Because the various devices and applications from different vendors need to operate with one another in a network, the OSI Model exists to give network vendors, developers, and engineers clear guidance on the various functions in a network and/or telecommunications system. Because this model allows for intercompatibility between vendors and their products, the OSI model was adapted by the International Organization for Standardization (ISO) as a model for networking in 1983.

The big ticket concept behind the OSI is this: the way information is communicated across the network can be divided into seven groups of functions called layers. Every layer in the OSI model is interdependent: each layer supports the layer above it, and is served by the layer below it. If two users were to send and receive a message, the data would first flow from the sending computer, then across the network, and finally through the different layers in the computer receiving the message.

Throughout the model’s seven layers, data is encapsulated and converted into a protocol data unit (PDU) that is digestible by the layer it is in. Across the seven layers, PDUs go from symbol form, to frame, to packet, to segment, and then data.

The seven different OSI layers include:

Layer 7: The Application Layer
The application layer is responsible for applications that send and receive data — for example, email.

Layer 6: The Presentation Layer
Not the most exciting layer, but a necessary one: the Presentation layer takes data and translates it into a format that its recipient can read (such as encrypting or decrypting information).

Layer 5: The Session Layer
The session layer is all about connections: this layer ensures that communication with remote systems can happen and that data can flow back and forth. SSH, SNMP, Telnet, and SSL are protocols used by this layer.

Layer 4: The Transport Layer
The transport layer is crucial for network security. This layer ensures that packets get where they need to go: its responsible for packets reaching their endpoints without errors and properly sequenced.

Layer 3: The Network Layer
The network layer is responsible for the way in which data is communicated both in your network and with other networks.

Layer 2: The Data Link Layer
The data link layer is responsible for making communication happen between the network and physical layers.

Layer 1: The Physical Layer
In the physical layer, raw data is transmitted and received by physical devices such as switches, routers, cabling, patch panels, and so on.

As this model for networking is, first and foremost, a theoretical model — it’s pretty rare that you would find it instituted exactly as it’s outline on paper. For a host of different reasons, network tools rarely keep all related IT functions together in one of the OSI model’s seven predefined layers.

If you’d like to find out what you should/can document and diagram for each of the above OSI layers, this article gives you both a history of the OSI model and more in-depth information for each of the seven layers.