Why the model helps.
THE IDEA
Networking is organised into layers, each handling one part of the job.
THE LAYERS IN PRACTICAL TERMS
- Physical: cables, wireless signals
- Link: communication between devices on the same local network
- Network: addressing and routing between networks
- Transport: reliable delivery between applications
- Application: the actual service, such as web or mail
WHY IT HELPS
It tells you where to look.
HOW TO USE IT WHEN DIAGNOSING
Work upward.
Is the cable connected or the wireless associated? Does the device have an address? Can it reach other networks? Does the name resolve? Does the specific service respond?
WHY UPWARD
A problem at a lower layer makes everything above it fail.
Checking the application first wastes time when the cable is unplugged.
WHAT MOST PROBLEMS ARE
Lower than people assume.
WHAT TO REMEMBER
Each layer depends on the ones below it.
WHAT THIS REPLACES
Guessing.
