Exercise 3: Local Devices Work, No Website Loads — Possible Solution
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Most likely cause:

Per the chapter's own warn-box, "if the default gateway is
misconfigured or down, every off-subnet destination becomes
unreachable -- while local, same-subnet traffic keeps working
perfectly fine, since it never needed the gateway in the first place."
The exact symptom described -- every local device reachable, but no
website reachable at all -- is the specific, named scenario the
chapter's own warn-box calls out directly: "I can reach my neighbor's
device on the same subnet, but not google.com." The most likely cause
is that the user's default gateway (their home router, per net1-4's
own example) is either misconfigured, offline, or otherwise not
functioning correctly.

Why local devices still work fine:

Per the chapter's own core explanation, "a device already knows how to
deliver a packet locally -- same subnet, direct Data Link-layer
delivery... Routing exists specifically for the moment the destination
isn't local." Reaching another device on the same home network never
requires the default gateway at all -- that traffic is delivered
directly at the Data Link layer between devices that are already on
the same subnet, with no routing decision needed. The gateway is only
involved once a destination falls OUTSIDE the local subnet, which is
true of every website on the internet (all of which live on entirely
different networks from the user's own home subnet).

Since websites are, by definition, never on the user's own local
subnet, reaching any of them requires the default gateway to hand the
traffic off correctly -- exactly the step the warn-box identifies as
broken. This is precisely why the symptom looks selective (some things
work, others categorically don't) rather than looking like a total
network outage: only the specific off-subnet path through the gateway
has failed, while the local subnet's own direct delivery mechanism
never depended on the gateway working at all.

WHY THIS WORKS AS AN ANSWER
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This identifies the default gateway as the likely cause using the
chapter's own named example almost verbatim, and separately explains
WHY local traffic is unaffected (it never routes through the gateway
in the first place) rather than treating "local works, remote doesn't"
as an unexplained coincidence.