Another device on the network is using your computers' IP address

HDFan

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Original poster
Jun 30, 2007
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So I'm getting this message when using a fixed DHCP .10 IP address for my iMac.

Screen Shot 2019-12-22 at 5.27.10 AM.png

1. It's simple, right, just make sure that the DHCP server on the network doesn't step on the address? Well it isn't, as the DHCP servers' starting address is .100. The only devices < 100 are those to which I have assigned fixed addresses (.01,.05,.20, .30).

2. So somehow someone else is getting the address then. I do a network scan from my iPhone. It only shows my iMac at that address. I look at my DHCP router. It shows my iMac - with the name iMac-2, at that .10 address. It is if my iMac is stepping on itself, seeing it's own address as a conflict.

3. Other devices with fixed IP addresses < 100 have no problems.

4. The message goes away, of course, if I set the iMac up to use DHCP. But in contrast to other devices on the network, such as NASes, which retain their DHCP served address for weeks, the iMac keeps getting a new address every few hours when continuously powered on.. This causes chaos with things like Plex servers.

So I am at a loss as to where to look ...
 

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Soba

macrumors 6502
May 28, 2003
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290
Rochester, NY
So I'm getting this message when using a fixed DHCP .10 IP address for my iMac.

View attachment 884209

1. It's simple, right, just make sure that the DHCP server on the network doesn't step on the address? Well it isn't, as the DHCP servers' starting address is .100. The only devices < 100 are those to which I have assigned fixed addresses (.01,.05,.20, .30).

2. So somehow someone else is getting the address then. I do a network scan from my iPhone. It only shows my iMac at that address. I look at my DHCP router. It shows my iMac - with the name iMac-2, at that .10 address. It is if my iMac is stepping on itself, seeing it's own address as a conflict.

3. Other devices with fixed IP addresses < 100 have no problems.

4. The message goes away, of course, if I set the iMac up to use DHCP. But in contrast to other devices on the network, such as NASes, which retain their DHCP served address for weeks, the iMac keeps getting a new address every few hours when continuously powered on.. This causes chaos with things like Plex servers.

So I am at a loss as to where to look ...
What happens when you change your system to use DHCP, then ping the .10 address from Terminal? Do you still get a response to the ping?

My first guess is that you unknowingly have a second device on your network that has a DHCP server running and is distributing IP addresses. Workstations won’t do this without 3rd-party software installed, but you mentioned a Plex server, so have a close look at that or any other such server. If you have multiple wireless access points on your network, also check those to ensure just your router is distributing IP addresses.
 
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HDFan

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Jun 30, 2007
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My first guess is that you unknowingly have a second device on your network that has a DHCP server running and is distributing IP addresses.
I do have another wireless router on the network. DHCP is off, but I'll need to run a test with it powered off. l can't do it right now as I have a lot of HomeKit Satechi power switches connecting to that router. After Comcast installed my new router it kept going down every few days taking down HomeKit so I installed another router so when it went down HomeKit didn't go down as well. Router/modem is now stable so I'll migrate stuff over.

Put a reverse firewall like from the shareware app called Little Snitch and you will be surprised at all the “call home” all the time!
Yeah, thanks. I have Little Snitch but it doesn't show anything.
 

Soba

macrumors 6502
May 28, 2003
269
290
Rochester, NY
I do have another wireless router on the network. DHCP is off, but I'll need to run a test with it powered off.
Without knowing which device is assigned the .10 address on your network, powering off the second router probably won't tell you anything; a system that receives an address via DHCP will hold onto it for an arbitrary period of time. Most modern DHCP servers require address lease renewal every 8 hours by default, but it could be much longer (days or even weeks). The mystery system will not relinquish its address until this lease has expired, so you'd have to leave it off for an extended period of time.

When you receive the error message, assign your Mac a different IP temporarily, then ping the .10 to see if you get a response. I'm quite certain you will! Then, check your router (or any managed network switches, if you're lucky enough to have such devices on your network) to see if it will tell you which port a device is connected on; this should give some clues about the location of the mystery device by letting you match its IP or MAC address to the physical port on the router or switch.

If your router and switches don't provide this information or if you are not sure how to do this, then it's a matter of checking every single device on your network to find out which ones are either assigned the .10 as a static IP (perhaps you forgot you statically assigned something else this address some time ago) or there's a second DHCP server lurking somewhere.
 

HDFan

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Original poster
Jun 30, 2007
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My Comcast router lease time is 1 week. It never served addresses in the .10 range (>+50 was used). The other router's DHCP has been turned off for months. When it was working it was set to >= 100 with a lease time of 120 minutes.

I have looked up active addresses on the network when this error occurs with lanscan and inet network scanner. Both show the same thing, my iMac both at the .10 and at the new DHCP address. No other devices ever were using that .10 address. Didn't ping that .10 address though. Occurrences seem to be becoming less frequent but when it happens I'll try the ping.
 

BrianBaughn

macrumors 604
Feb 13, 2011
6,538
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Baltimore, Maryland
This definitely isn't a new issue as there are probably questions posted on the internet going back over ten years on it. Having said that, I don't remember what the causes could be or if there's a universal fix. Maybe some sort of animosity between the Mac and the router's firmware.

If you finally get this figured out…great. However, I would advise setting the IPs of all your devices with the router's firmware using "DHCP Reservations" or (whatever the firmware wants to call it) and turning off the static IPs set on devices.
 

Soba

macrumors 6502
May 28, 2003
269
290
Rochester, NY
This definitely isn't a new issue as there are probably questions posted on the internet going back over ten years on it. Having said that, I don't remember what the causes could be or if there's a universal fix. Maybe some sort of animosity between the Mac and the router's firmware.

If you finally get this figured out…great. However, I would advise setting the IPs of all your devices with the router's firmware using "DHCP Reservations" or (whatever the firmware wants to call it) and turning off the static IPs set on devices.
@HDFan I second this. If your router supports them, use reservations.

This point aside, I suspect one of your network switches (or the router itself) has a hardware issue.
 

HDFan

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Original poster
Jun 30, 2007
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OK, found it. But still have the same problem. On boot the IP address is set to ... 100. But at some random time it changes. The assigned IP address changes at the same time. Since it changes automatically don't get the duplicate address error.
 

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BrianBaughn

macrumors 604
Feb 13, 2011
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Baltimore, Maryland
I'm not sure I understand. You have set the reserved IP on your router's firmware…which is correct.

Is your iMac, at System Preferences>Network set to "Automatic" at Configure IPv4? It should be.

Additionally, no other devices should be assigned that IP manually..
 

HDFan

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Jun 30, 2007
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Configure IPv4 in Network Preferences on the Mac is set to "Using DHCP". IPv6 is set to "automatically".

No other devices are manually assigned that address
 

ArrayDecay

macrumors member
Feb 21, 2019
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Greater Seattle Area
I occasionally get this warning and have learned to ignore it. Probably not a "best practice" but I couldn't ever track it down and it seems harmless. It's existed over several generations of Apple routers.