The letter with no road
Imagine you post a letter to a village. The sorting office has no road to that village on its map. The letter goes nowhere, and you get a note back.
A router (a device that passes data on) behaves the same way. With no route, it drops the packet (a small piece of data) and sends back a note.
Today you add the missing roads.
Key words
| Word | What it means |
|---|---|
| Network | computers and devices joined so they can share data |
| Router | a device that passes data between networks |
| Packet | a small piece of data sent over a network |
| Route | the road a packet takes to reach another network |
| Routing table | the list of routes a router knows |
| Next hop | the next router a packet is handed to on its way |
| Static route | a route a person types in by hand |
| Default route | the road used when no other route fits |
| Traceroute | a tool that lists each router a packet passes through |
| Subnet | one smaller network that is part of a bigger one |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can configure a static route on a router, test it, and save it.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Two routers that know only their neighbours
Each router knows the networks it is plugged into. That is all.
Router r1 knows 10.0.1.0/24 and 10.0.12.0/24. It has never heard of 10.0.3.0/24, where h2 lives. So h1 cannot reach h2 yet.
The lab is a small network built inside one machine. Press Start the lab, then try the first ping.
Ping h2 from h1. It will fail.
sudo ip netns exec h1 ping -c 2 -W 1 10.0.3.2A command for the lesson’s lab terminal. It is shown here and not run.
Read the failure
Note
The line says "Destination Net Unreachable" and it comes from 10.0.1.1. That is r1 telling h1: I have no route. The packet did not get lost on a cable. The router said no.
See where it stops
sudo ip netns exec h1 traceroute -n -m 3 -w 1 10.0.3.2A command for the lesson’s lab terminal. It is shown here and not run.