The OSI model is away of describing how different applications and protocols interact on network-awaredevices. We explain the role of each layer and of the stack. | Tutorial:Overview Understanding The OSI 7-Layer Model The OSI model is a way of describing how different applications and protocols interact on network-aware devices. We explain the role of each layer and of the stack. By Neil Briscoe f you spend much time in the com- on a slightly modified layer system. governed by a device’s MAC address, pany of network technicians you TCP/IP, for example, uses a 6- rather the six-byte number that is unique to Iwill eventually hear them say than a 7-layer model. Nevertheless, in each NIC. Devices which depend on something like “That’s Layer 2 only” order to ease the exchange of ideas, this level include bridges and or “That’s our new Layer 4 switch”. even those who only ever use TCP/IP switches, which learn which segment’s The technicians are referring to the OSI will refer to the 7-layer model when devices are on by learning the MAC (Open System Interconnection) Refer- discussing networking principles with addresses of devices attached to vari- ence Model. peers from a different networking ous ports. This model defines seven Layers background. This is how bridges are eventually that describe how applications run- Confusingly, the OSI was a work- able to segment off a large network, ning upon network-aware devices ing group within the ISO (Interna- only forwarding packets between may communicate with each other. tional Standards Organisation) and, ports if two devices on separate seg- The model is generic and applies to all therefore, many people refer to the ments need to communicate. Switches network types, not just TCP/IP, and model as the ISO 7-layer model. They quickly learn a topology map of the all media types, not just Ethernet. It is are referring to the same thing. network, and can thus switch packets for this reason that any network tech- Traditionally, layer diagrams are between communicating devices very nician will glibly throw around the drawn with Layer 1 at the bottom and quickly. It is for this reason that mi- .