Everything is down, and you cannot guess
A desk cannot reach the server. The cable could be cut. The address could be wrong. A device could be missing a setting. Something could be blocking the way.
Guessing wastes hours. The fast way is to test from the nearest device, one step at a time, and let each answer tell you where to look.
By the end of this lesson
Note
I can find the cause of a fault by testing outwards from the nearest device.
Key words
| Word | What it means | Example |
|---|---|---|
| Gateway | the router a device sends messages to when the target is on another network | 10.0.3.1 |
| Route | a rule that says which way to send messages for one network | |
| Default route | the rule used when no other route fits | |
| Link | the connection between two devices | |
| Symptom | what you see that tells you something is wrong | |
| Root cause | the real reason for a fault, not just what you see | |
| Network | computers and devices joined together so they can share data | |
| Router | a device that passes messages from one network to another | |
| Firewall | a device or program that blocks some messages and lets others through |
The cells students fill in are left blank in this preview.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
This is practice
Important
In the real Task 2 you test a network in the exam board's own simulator. Here you use real commands in a lab. The method is the same: test, compare, decide, change one thing, test again. Your teacher will tell you what tools the real task gives you.
Worked example: what each message means
| What ping or the system shows | What it usually means |
|---|---|
| Network is unreachable | This device has no route to that network. Look at its link and its default route |
| Destination Net Unreachable, from a router address | That router has no route. Look at the route table on the router that replied |
| 100% loss and no message, but the gateway replies | A wrong gateway further on, a missing return route, or a firewall |
| 100% loss, even to the gateway | A link or address problem on this device |
The cells students fill in are left blank in this preview.
Set up: give router A a route
sudo ip netns exec r1 ip route replace 10.0.3.0/24 via 10.0.12.2A command for the lesson’s lab terminal. It is shown here and not run.
Set up: give router B the way back
sudo ip netns exec r2 ip route replace 10.0.1.0/24 via 10.0.12.1A command for the lesson’s lab terminal. It is shown here and not run.