Linux Firewalls, Third Edition

The OSI Networking Model

The OSI (Open System Interconnection) model represents a network framework based on layers. Each layer in the OSI model provides distinct functionality in relation to the other layers. The OSI model contains seven layers, as shown in Figure 1.1.

Figure 1.1. The seven layers of the OSI model.

The layers are sometimes referred to by number, with the lowest layer (Physical) being layer 1 and the highest layer (Application) being layer 7. If you hear someone refer to a “Layer 3 switch,” he is referring to the third layer of the OSI model. As a person interested in security and intrusion detection, you must know the layers ...