The OSI Model

The most popular network communication model by far is the 7-layer Open Systems Interconnect (OSI) model, designed by the ISO in the 1970’s. While today we might take for granted the ability to communicate between different systems, this was not always easy or possible. In the golden days of computing, equipment from IBM couldn’t talk to that of Digital, making interoperability difficult to impossible. The goal of the ISO was to create a reference model that would clearly define network functions and responsibilities, ultimately allowing different systems to interconnect and communicate. It is worth noting that the OSI is primarily a reference model, although an actual implementation of an OSI protocol suite does exist. In reality, most network protocol stacks tend to map to the model roughly, but seldom exactly.

Figure 1-1: The OSI Model and associated layer numbers.

Each of the seven layers is known not only by a name, but also by a number. To that end, you’ll often hear the Network layer referred to as Layer 3, or the Transport layer as Layer 4. Remembering the layer names and numbers is critical. Many people get these layers confused in the beginning, so consider using the first letters of each layer to create a mental reminder. Of these, the most popular is probably “All People Seem To Need Data Processing”. If that doesn’t work for you, make up one that will help you remember the order of the layers easily.

Tip: To remember the names and order of the layers in the OSI model, use the first letter of each layer to create a mental reminder (mnemonic), such as All People Seem To Need Data Processing.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.