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I have assigned static IPs in my router (Sercomm H300) for almost all the devices in my network, smartphones, tablets, smart WiFi switches e.t.c. There are times that many devices are not visible to my computer (I cannot ping them and I don't see them using 'arp -a'). In my router I can see them as connected. If I start using a device like a smartphone, it becomes visible to my computer.

I use a raspberry pi running Home Assistant and the weird thing is that the raspberry pi can access some of the devices that my computer can't (but not all of them). Last time I checked I couldn't ping 7 out of 9 WiFi switches and the remaining two are unavailable in Home Assistant. From my computer I can ping only 5 out of 9. How could a device be connected on WiFi and not being visible to the computers? How could be explained that some devices are visible to one but not another computer? All the WiFi smart switches are the same brand and were configured with the same app.

I also have setup a notification when all of my WiFi switches are unavailable and I get that notification periodically indicating that WiFi drops down momentarily. What could cause this?

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    See if your router supports DHCP reservation. That is how I give my iPhones static IP addresses. No issue seeing them.
    – anon
    Commented Jan 25 at 15:31
  • When you configure a specific IP address on the client, that is a "static IP". When you assign a specific IP address TO a client on the DHCP server (likely your router in this setup), that is a "reserved IP" or an "IP reservation". Can you clarify how you have set these up? Confirm that, in a normal scenario, without these static (or possibly reserved) IPs, the IP addresses used by devices in the network come from this router and not another device, such as a modem. Also, make sure that your router is not configured to isolate devices from each other.
    – music2myear
    Commented Jan 25 at 15:45
  • I used IP reservation from my router. I don't have admin privileges on my router so I think I don't have the option to isolate the devices even if I wanted to. The ISP has the admin credentials and they don't distribute them.
    – John
    Commented Jan 25 at 17:45
  • What could I check to debug this? I notice that I cannot ping other devices that are definitely connected on the internet, like a firestick tv even while it's streaming. After restarting the firestick I can ping it. THe other devices like the smart switches cannot be turned off and on unless I turn off the circuit breaker.
    – John
    Commented Jan 25 at 17:56
  • What are these IPs and also the local IP of the router? What is the netmask?
    – harrymc
    Commented Jan 25 at 20:56

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A common cause of the symptoms you described is when Wi-Fi group keying (i.e. encryption of multicast/broadcast packets) breaks on some devices, breaking multicast and broadcast, which breaks service discovery protocols and ARP. Rebooting affected devices or forcing them to leave and rejoin the network usually puts things right for a little while until they hit the bug again.

Group keying on Wi-Fi is tricky and there are a lot of buggy implementations out there that break from time to time. The trickiest part that breaks the most is when you have original WPA (TKIP) available as an option in addition to WPA2 (AES-CCMP). So go into your AP's (wireless router's) settings and make sure it's set to WPA2-PSK only (AES-CCMP only). You don't want original WPA (TKIP) available as an option. You want it completely off.

A second issue that can break multicast and broadcast (and thus breaks discovery and ARP) is your multicast data rate being set too high, making it hard for some devices to receive multicasts and broadcasts reliably. If your AP lets you set your multicast rate or enable or disable older/slower data rates, make sure all rates are available and your multicast rate is set low.

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    I changed TKIPandAES to AES and I the problem fixed immediately. That was the issue indeed. Thank you
    – John
    Commented Jan 26 at 10:57

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