Sorry I may have communicated this poorly.

We were connected to the local area network, but we had no Internet access (which was behind a paywall). However, only being connected to the local network was sufficient for Briar -- no Internet needed!

To be fair, it would absolutely be possible to set up a hotspot where all the clients were jailed and invisible to each other, had IPs in different subnets, whatever.

Conventionally, I guess that sort of isolation doesn't typically happen as it's more work to set up and only really benefits users (privacy from each other), but maybe if more apps figure out how to take advantage of this things could change.

It’s not really more work; it’s just a matter of enabling the ‘client isolation’ setting on the wireless AP.

For example, see Cisco’s Meraki documentation: https://documentation.meraki.com/MR/Firewall_and_Traffic_Sha...

> With Client Isolation enabled, clients will only be able to communicate with the default gateway and will not be able to communicate with any other devices on the same VLAN (or broadcast domain). In order for the wireless client to communicate with another device, the upstream gateway must be used to enable this communication (e.g. inter-VLAN routing and ACLs). Any traffic bound for an address on the same VLAN as a device in client isolation will be denied. Traffic bound for other VLANs will be forwarded and routed normally.

Even that extremely minor step counts as “more work” for the clientele we’re talking about here.

I would certainly expect a cruise-ship-wide paid network to be managed professionally.

Realistically though, it's probably a multi-ship contract involving some small IT firm who installs everything, and then gives the onboard electrician/maintenance-guy a quick primer on how to do basic diagnosis of problems.

I'd bet that most of the configuration decisions are made based on how to minimize the likelihood of such problems ever occurring.

But you can't use Briar and the like with this Client Isolation, can you?

Isn't that their point? The only reason it worked on a cruise ship connected to the WiFi was because local devices could talk to eachother and were not isolated.