This isn't strictly a Linux issue, but ...

My home network has a Win10 laptop, an iPad, an Android phone, and a RHEL9 system.

* All systems are in the 192.168.0.* subnet.
* All systems can reach the Internet through my router.
* Most systems can't reach SOME other systems:

iPad: can ping all systems
Linux: can't ping iPad
Android: can't ping Windows
Windows: can't ping iPad or Linux, CAN ping Android !?

I don't see any pattern here. Any guesses what might be causing it, and how I can diagnose / fix it?

Thanks!

Ping is a poor test.

Since all of them can reach the internet then I'd consider this to be a firewall issue.

One some home routers they jail the connections between wired and wireless.

Map the network and see who is listening to what.
See:
man nmap

Also see what the router is doing with what machines:
Then fix whatever setting is causing it.

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Yes ping is crude, but it's a simple first test.

If it's an external firewall (such as the router), wouldn't it divide the systems into two unconnected groups, e.g. wired/wireless? As you can see from the connectivity diagram, there's not a clean division like that.

If it's firewalls in the systems, wouldn't that require firewalls in all non-contactable systems? I.e. Windows, iPad, and Linux? Android is the only one that all other systems can reach.

I drew a diagram of "what systems can ping what other systems" to visualize it better -- below.

As you'd expect, it's one hop from all systems. They're all connected directly to the router via wifi or (for Linux) LAN.

Unfortunately nmap is not on my system, and I can't get the installer (yum) to work. Still poking at it.

Attached Thumbnails

 

Windows' firewall is on by default. If you turn it off, you get nagged to turn it back on and sometimes it turns itself back on when you're not looking. Have you turned it off?

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I hadn't, but I tried it. I turned off the Windows Defender firewall for the local private network. Only change I see is that the Linux system can no longer ping Windows. But that may have been an error before -- I still can't ping Linux -> Win after turning the firewall back on.

You are telling nothing about you network setup, but I presume most of your devices are connected via wi-fi and if it is at least dual-band. Some buggy routers have issues with communication between bands, try locking all your devices to a single band. Also many routers have a wireless isolation security option which intentionally prevents wireless devices from communicating with other devices on the network - check configuration of your router to see if it is enabled.

The Linux system is connected via Ethernet LAN. All others (Win, Android, iOS) are on 5G IPv4 wifi.

The router (Arris TG3452A) has a firewall, but it's set to minimum security (nothing blocked). It supports "Home" and "Guest" networks. Each has an AP Isolation setting, which is currently On on the Guest network, Off on the Home network (where I work). I disabled it on both networks and saw no change.

Guest network by design usually blocks inter lan access.

Sometimes just turning these features off is insufficient. You may need to reboot the devices to get the new settings to work.

What does arp -a tell?

Your Android phone and iPad may be configured for random MAC addresses when then connect which if you did not always check could cause part or your problem and I think that there is an iPad setting to regenerate a new mac every 24 hours.
Obviously the desktop would not change and then compounded by if you are using the guest access point for some devices.

PING is a poor test, and one used by so much malware that some systems have the ECHO protocol it depends upon blocked. There is also a network "stealth mode" that prevents an "echo" reply. I recommend using several tools, not all based upon ping.

I assumed it probably did. But I don't use it. My housemate uses that network.

Attached below.

That would not explain Windows' inability to reach Linux, would it?

I used several tools on Win/iOS/Android/Linux, but I think most of them were using ping. What tools would you suggest? I still haven't resolved the yum install issue on Linux so I have limited options there.

Interesting. When I did my tests yesterday, Windows and Linux could not reach iOS. I just did a test ping from Linux to iOS and it hung up. I left it runnning -- and several minutes later the pings started working. Now Windows can reach iOS too.

arp -a output:

You can verify if MAC spoofing is to blame by deleting arp entry for inaccessible device (arp -d).