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I have a laptop and a tablet that are both attached to a home router. Often I copy files to/from the tablet using Wifi.

But sometimes it cannot make the connection. I downloaded some "lan scan" programs to try to see what is happening. It turns out: - The laptop at 192.168.0.3 can see the router (and can surf the internet) but the network scan from the laptop does not see the tablet. (I used the LanScan program on OSX). I also tried ping'ing the tablet ip from the laptop, which failed.

And the tablet at 192.168.0.4 can see the router (and can surf the internet) but the network scan does not see the laptop (or anything) at 192.168.0.3.

What would cause this?

Restarting the router has fixed this (at least twice), but I would like to know what is going on (if it is possible to understand - I'm a bit of a programmer, but know little about networking)).

Here are screenshots from the scan on the tablet:

tablet lan scan

And from the laptop:

laptop lan scan

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  • I assume outside of the times it cannot make the connection that it actually can make the connection? I am confused you have an Apple device on both 192.168.1.3 and 192.168.1.4 identify which device is which.
    – Ramhound
    Commented Aug 24, 2017 at 12:55
  • Why don't you just use AirDrop to copy files between your Macbook and your iPad Pro? This way you make a direct connection between the two, bypassing the wireless infrastructure.
    – pythonian
    Commented Aug 24, 2017 at 17:00

1 Answer 1

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It sounds like the Wi-Fi AP functionality of your router is buggy and stops handling multicasts correctly until it gets rebooted. Or it could be your laptop or tablet with the bug, but if those are both Apple products it's not likely, as Apple cares about multicast a lot and does far better than the industry average at not screwing it up.

Discovery protocols such as Bonjour make extensive use of multicasts, so broken multicast means broken discovery.

You can test this hypothesis by pinging the "all hosts" multicast address, 224.0.0.1. Do it from the laptop and make sure you see responses from the tablet, and then do it from the tablet and make sure you see responses from the laptop. Do this when discovery is working and make sure it works in both directions. Then the next time you notice discovery has broken, do it again in both directions and see if it stopped working in one or both directions. If it breaks, it confirms that multicast in general has broken, not just your discovery protocol.

One Wi-Fi AP setting that no one really needs, and that makes multicasts needlessly complex and more likely to expose bugs, is leaving original WPA (TKIP) enabled on a WPA2 (AES-CCMP) network. Sometimes this is called "WPA2 mixed mode" or "WPA/WPA2". So make sure that your AP is set for pure WPA2 only (AES-CCMP only) and you'll be less likely to see bugs that break multicast.

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  • Yes, this describes the problem. I did the test of "ping 224.0.0.1" when it was/was not working, and yes, when not working the tablet does not show up in the ping. (Both devices are apple, yes)
    – bull
    Commented Aug 25, 2017 at 14:36
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    Your remark here about WPA2+WPA mode helped me solve a similar problem to the one in the OP. Switching the router from WPA2+WPA to just WPA2 got all the devices talking to each other. Thanks for the idea!
    – Gregory J. Puleo
    Commented May 12, 2021 at 21:22

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