Choose a diagnostic by its question

An interface joins a host to a network. A route describes where traffic
goes toward a destination. A socket is a communication endpoint; a
listener waits for incoming traffic. ICMP, Internet Control Message
Protocol, carries control and diagnostic messages such as echo requests.

The commands below name jobs, not observed output from your machine.
For each card, separate the observation from any possible explanation.
A missing echo fits several causes; a listener is not a page response.
Use the printed records without sending any probe.

Command jobs for this lesson. The blocks below are not captured command output.

ip address show
Job: list IPv4 and IPv6 addresses assigned to interfaces.
The Ubuntu noble ip-address manual uses this command as an example.
A later run on your machine will show your interfaces. Do not copy those addresses into the question.

ss -ltn
Job: show listening sockets, limit the list to TCP, and print numeric ports.
This inspects sockets on the machine. It does not send a probe to another host.
Do not add -K. That option attempts to close sockets.

ping -c 2 127.0.0.1
Job: send two ICMP echo requests to the IPv4 loopback address, then stop.
Addresses of the form 127 and anything must not appear outside the host, so this probe stays on the sender.
This page shows no captured reply. A later run may print replies and times. Those lines were not recorded here.
If the command is refused, stop. Do not change permissions.

Activity listener card, fictional, not ss output
State   Local address
LISTEN  127.0.0.1:9090
Predict before reading the check. Check: the listening TCP port on this card is 9090.

Failed-echo card, fictional model. Do not send this probe.
Drawn probe: ping -c 2 203.0.113.10
Drawn result: no echo reply
Two hypotheses that fit the card, neither proven by the drawing:
1. The echo was filtered.
2. The sender had no route to that address.