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Another device on the network is using your computer's IP address...

I get this alert several times a week:


This situation lasts a few minutes, then recovers by itself. At least three Macintosh computers on our LAN, each running a different operating system, get this message and experience this effect, then recover in a few minutes, a few times a week.


While one computer is experiencing this situation,


    1. The affected computer is not connected to the web, but all other computers remain connected and seem to work fine.
    2. The WiFi symbol in the screen-top menu bar of the affected computer displays a dimmed icon with a bang "!" over it.
    3. The affected computer can connect via WiFi to the web via another path, such as an iPhone, and that works OK.
    4. After a few minutes, the situation clears: The affected computer's WiFi icon in the menu bar returns to normal and its web interface returns. The alert box stays up, however, so we'll know that it did happen.


Other non-computer devices on our LAN (Roku, Echo, iPad, garden sprinkler timer) also seem to experience a web access outage in a similar pattern: for a few minutes a few times a week. Again, their experiences seem to occur at unique times, while all other devices and computers on our LAN continue uninterrupted web access in the meantime.


Our LAN is WiFi-only; no cables. About 15 devices connect to the web (and sometimes each other) via a DSL modem/router and one extender. Most of the time it works fine.


I'd love to ask our ISP to try replacing our DSL modem, but, by the time I were to initiate a service call, the situation would have cleared.


To help me research this problem, I have two questions:


    1. Where are logs stored where I can see timestamps for when this alert came up?
    2. How does macOS learn of this problem? Does the modem somehow tell it?


Your suggestions and speculations will be welcome.


--Gil

MacBook Pro 13″, macOS 10.14

Posted on Aug 3, 2022 9:37 PM

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Question marked as Top-ranking reply

Posted on Aug 4, 2022 11:12 AM

Have you tried power cycling your router (or modem/router)? Or checked your router's logs and configuration?


The macOS system logs are pretty much useless for troubleshooting these days. Normally you can use the Console app to locate & view the various system logs, but Apple thinks so highly of these logs that many of the logs no longer appear within the Console app. Good luck trying to find anything useful in them. The logs are located at these locations:

  • /Library/Logs/DiasgnosticReports
  • /private/var/log (I also see /private/var/logs on some systems which may contain a few entries --notice the "s" for some app logs as Apple cannot even be consistent in their naming of the folder -- but not sure what is using it as I ignore it)
  • ~/Library/Logs



Similar questions

8 replies
Question marked as Top-ranking reply

Aug 4, 2022 11:12 AM in response to Gil Dawson

Have you tried power cycling your router (or modem/router)? Or checked your router's logs and configuration?


The macOS system logs are pretty much useless for troubleshooting these days. Normally you can use the Console app to locate & view the various system logs, but Apple thinks so highly of these logs that many of the logs no longer appear within the Console app. Good luck trying to find anything useful in them. The logs are located at these locations:

  • /Library/Logs/DiasgnosticReports
  • /private/var/log (I also see /private/var/logs on some systems which may contain a few entries --notice the "s" for some app logs as Apple cannot even be consistent in their naming of the folder -- but not sure what is using it as I ignore it)
  • ~/Library/Logs



Aug 4, 2022 7:09 AM in response to Keith Barkley

Thank you, Keith, for the helpful speculation. However, the scenario you describe does not fit the evidence. This situation sometimes occurs even during the night, while LAN activity is quiescent, all computers are up and running, none is offline, no devices are being used, and there is no purpose for any computer or device to join or go offline.


If I could get my hands on our Macs' log showing timestamps for this alert, perhaps I could find a pattern.


Yes, it is benign, in that the situation repairs itself in a few minutes. And then it typically occurs again in a different computer or device for an additional few minutes' outage. Benign, perhaps, but annoying, with the following effects when this happens on a computer that one is using:


    1. The movie one is watching inexplicable and without warning pauses for a few minutes displaying the Orabora.
    2. The article that you're reading cannot link until this situation passes.
    3. Monitoring-type software shows a gap in its logs.


Granted it is benign, but I still hope to find a solution. We first noticed this behavior within a week after our ISP replaced a modem that had quit working entirely. Now I think that our current modem exhibits an intermittent failure. However, I feel that I need some evidence before I ask our ISP to try another modem.


Please, where is that log?


--Gil

Aug 5, 2022 3:52 AM in response to HWTech

This looks like you have two DHCP servers in your network. A DHCP server hands out IP addresses to other devices. When you have more than one such DHCP server a device requesting an address is served by the one with the faster reaction. This can (and often does) result in an address collisions. As to "everything is stable during the night". Well, a computer or other dynamically configured device (like an access point or a phone) is trying to renew its address on a more or less regular base. This is no problem if it is always the same DHCP server answering, but sometimes the other one is a micro second faster - and boom, address collision. Maybe the new modem is configured to act as a DHCP server but you already have something else setup for that purpose. Check the configuration of your modem and other network devices. You must have only one DHCP server in your network otherwise you will have exactly the issues you see.


Happy hunting!

Aug 5, 2022 1:15 PM in response to ruggiero

Thanks, HWTech--


I have tried power recycling our DSL modem, with no noticed effect.


You are quite right that the macOS system logs are difficult to interpret. I managed to reduce the clutter a bit by searching for files modified in the last 5 days.


Thanks also for suggesting our modem's internal logs. I had not looked there! Yes, there's stuff!


I don't have an answer yet, but now I have something to work on.


Thanks.


--Gil



Aug 5, 2022 1:21 PM in response to ruggiero

Thanks, ruggiero, for your suggestions--


I can't think of any device in our network which would be serving as a competing DHCP server . As we have 15 devices connected to our DSL modem via W-Fi, though, it's certainly a possibility. Thanks for your suggestion.


Also thanks for mentioning that some gadget might be renewing its DHCP lease on a regular basis. Indeed, looking at our DSL modem's internal configuration, I can see that each DHCP lease expires in two hours. I had no idea that all this was going on while we were asleep!


Thanks for your suggestions. This will give me something to work on.


--Gil


Aug 9, 2022 8:55 AM in response to Gil Dawson

There's a lot more to this, and it will take some troubleshooting to identify.


First off, every device on your local network needs a unique IP address.


There are two common ways devices can get an IP address:


Firstly, devices can be configured manually, by giving each device all the details it needs to get online (IP address, subnet mask, router address, etc.).

Secondly, devices can query a DHCP server to get those details, and the DHCP server keeps track of which device has which IP address, so it knows what addresses are available vs. in use.


Most people use DHCP because it's easy (most people don't understand those manual terms anyway); those details are repetitive; tracking which device has which IP address; and change is hard (having to walk through every device on the network to make changes).


If *everything* on the network is configured to use DHCP, and there's only one DHCP server, the DHCP server knows which IP addresses have been issued, and this problem should not occur.


So this kind of problem occurs because of one of two conditions.


a) one device on the network is configured with a static IP address (in this case, 192.168.254.12). It doesn't care (or know) that a DHCP server is trying to hand out that IP address, and the DHCP server doesn't know about the statically-configured device, and so the conflict message appears.

b) there are two DHCP servers on the network that both think they are responsible for the 192.168.254.12 address. Neither of them know about the other server, so the second server doesn't know the first server has already handed out the address.

Note: there is NO typical way to control which DHCP server responds to a request on the network - when a device connects to the network, it sends out a DHCP request; any DHCP server on the network sends a response, and the first response the device sees is what it uses. For this reason, running multiple DHCP servers on a single network is not recommended, unless you're a senior network administrator and really know what you're doing.


So, in order to resolve this, you're going to need to identify the other machine on the network that's triggering the issue - either a duplicate server, or a rogue client.

Timing may be your best friend - try to identify what other devices were just turned on, or if someone just walked into the office with their laptop, etc.

If you have access to the DHCP server, its logs should indicate what IP address was handed out to each device, so that might help identify the culprit.

You *might* find some useful data in the OS logs, but that won't be more than a MAC address (although this might be useful in at least identifying the type of device you're trying to track down).

Since the problem is on WiFi, it could be some rogue user connecting to your WiFi network, so check your WiFi settings, maybe changing the WiFi password, or checking the WiFI logs to see what devices are connected (the ease of doing this will depend a lot on your WiFi router)



Aug 24, 2022 3:09 PM in response to Gil Dawson

Ah-Ha! I found it!


Your very helpful suggestions and persistence caused me to consider my NetGear EX6100v2 Range Extender, so I posed this same question on the Netgear Community Board. Sure enough! They have to resort to either reserved or fixed IP addresses to make this work.


Turns out that, during configuration of the EX6100v2, I had checked the box, "Same SSID as the Cable Modem's Router"? I figured, "Why not?" Well, this problem is exactly why not. I unchecked the box and the problem disappeared. My devices now either select the stronger SSID automatically, or I have set them to use the proper SSID depending upon where they are located.


I had wondered how NetGear's EX6100v2 could coexist with my Cable Modem's router, and now I know. The answer is, "Fine, most of the time, but with outages on the order of a few minutes a few times a week."


Separate SSIDs solves the problem.


Thanks for y'all's help.


--Gil

Another device on the network is using your computer's IP address...

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