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I have a Ubee cable modem (DOCSIS 3.0) which also serves as a wired/wireless router and DHCP server. I have devices which are on the LAN and receiving addresses from the Ubee that I can't ping from other devices also on the LAN.

Example:

  • Computer A (static IP 192.168.0.50 wired connection)
  • Computer B (DHCP IP 192.168.0.83 wireless connection)
  • Computer C (static IP 192.168.0.203 wireless connection)
  • Ubee (static IP 192.168.0.1)

Results:

  • Computer A can ping Ubee and Computer C (Can't ping B)
  • Computer C Can ping Computers A and B and Ubee
  • Ubee can ping B and C, but not A which is wired to the Ubee and shows up on the client list

(Computer B is a microcontroller and is working fine with MQTT)

Hope I have explained this well enough. It's driving me crazy. Is it something about the Ubee, something dumb I'm doing? Thanks for any help. Bob

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2 Answers 2

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The most likely explanation is simply different kinds of ping and firewalls. Pinging can be done multiple ways, and it is entirely possible firewalls are blocking some types of ping but not others.

A good partial solution could be to examine the ARP tables of each device to see if it knows it's neighbours (after attempting to communicate with them). Because they are all directly connected this should work even where ping doesn't.

It is much less likely, but conceivable that the Ubee is faulty or unable to track the connections and is causing intermittent issues.

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Doing multicast and broadcast packet delivery on 802.11 networks is tricky, and many products are buggy in this area. These bugs that break broadcast break ARP, which relies on broadcast. That breakage often means some device can’t successfully use ARP to find the MAC address of the local IP address it wants to ping, so it can’t send the ping request because it can’t find a MAC address to put in the packet’s MAC-layer headers.

One way to see if this is the problem is to temporarily disable wireless security on the network and move all the wireless devices involved to within 3m of the AP. If pings work fine then, then your problem is probably buggy 802.11 multicast/broadcast implementations.

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  • The ARPs show what you would expect; unreachable IP addresses are not listed. I don't think the problem is not related to wireless; the main failure (IMHO) is that of the Ubee router not to be able to ping a wired machine which it lists as a client.
    – Bob Sculley
    Commented May 28, 2018 at 5:22

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