Intermediate5 lessons105 minutes
Networking: how your server is reached
Addresses, ports and connections: name the failing layer instead of declaring "the network is down".
"Connection refused" is not a firewall: it is an answer. This course takes "unreachable" apart — address, route, listener, filter — and gives one test per layer, so the failure has a name before you touch anything.
What you will be able to do
- Read what ip a and ip route actually show, including on a server behind a provider's NAT
- Tell who can connect to a service from the Local Address column of ss
- Distinguish a refused connection from a timeout, and what each one proves
- Localise the failing layer with a repeatable ladder of tests
Outline
- 01
The path to your server
Addresses, ports, and the two ways a connection fails.
- 02
When the server is unreachable
A ladder of tests, and the classic case it catches.
Technical content last reviewed on 9 August 2026