OSI Model Tutorial

Welcome to the most basic tutorial for networkers! Understanding the OSI model is one of the most important tools for grasping how networking devices such as routers, switches and PCs actually work.

Think of a real-life example: sending a letter to a friend. To do it, you go through a series of steps:

1. Write your letter
2. Insert it into an envelope
3. Write the sender and receiver information on the envelope
4. Stamp it
5. Go to the post office and drop it into a mailbox

This example shows that we go through a set of steps, in a specific order, to complete a task. The same is true when two computers communicate: they follow a predefined model, called OSI, made up of seven steps, or layers.

This is the well-known table of the OSI model, worth learning by heart. A popular way to remember it is a sentence built from the first letter of each layer, for example All People Seem To Need Data Processing (top to bottom), or Please Do Not Throw Sausage Pizza Away (bottom to top).