Start from what the user sees
I ask what they tried, what changed, and when it last worked. Then I reproduce it myself.
Then narrow the layer
- Is it one machine or many? One points to the device. Many points to the shared part.
- Is it DNS, DHCP, or the link itself? A ping by IP versus by name tells me quickly.
- Did power, cabling, or a recent update happen right before?
I only change one thing at a time and test again. If I cannot explain why it broke, I have not fixed it yet.