The OSI model is designed to ensure compatibility of network devices (and their operating systems) regardless of manufacturer. OSI stands for Open Systems Interconnection and is defined by the International Standard Organisation (ISO).

OSI Model is used for easier learning and theory of networking but real life networks are based on TCP/IP Model.

The model has seven layers:

• Application layer
• Presentation layer
• System layer
• Transport layer - Manages reliability/congestion/connections/multiplexing.
• Network layer - Manages packets
• Data layer
• Physical layer - Physically how computers are connected

Which can be remembered by either of two mnemonics:

All People Seem To Need Data Processing

or from the bottom up:

Please Do Not Throw Sausage Pizza Away

Graphic showing the 7 OSI layers in detail, this is explained below.

The Host Layers.

The first 4 layers of the model really work at the operating system/software level, within the host: