Duplicate IP address?

[Stephen Watson]

Hi all,

My partner woke his MacBook from sleep and it came up with a message to the effect that "another device on this network is using the same IP address". He saw the same message about two weeks ago, but we can't remember if it was also when waking his Mac from sleep then.

We are using a Netgear modem connected to an Airport Extreme and all connected wirelessly. We also have two iOS device and another Mac connected. I used to see the same message myself when I was in Brighton every couple of months.

What does it mean? What causes it? Is it any cause for concern and if so, is there something we should do?

Thanks very much.

Stephen

Right. Way past my bed ... Zzzzz


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[John Patrick]

Morning,

It's a normal error, nothing usually to worry about especially if the
devices continued to work. In some situation it does indicate
networking issues, potential hacking/security but I would say from
what you have described it's more a warning and that it was
automatically fix itself.

From what you described I would expected something similar to the
following happened.

1) From what you describe I would say the device had been asleep for
more than 24 hours
2) A new device (new as in not seen on the network for more than 24),
was switched on


Tech Details Below For Those Interested...
-----

In standard consumer networking;
1) High level, computers usually talk to each other using host or
domain names. e.g. www.google.com

2) Low level, computers usually talk to each other using IP address
e.g. 74.125.132.104 (www.google.com current IP advertised to my laptop
just now)

3) Hardware level, devices (computer/servers/routers/switches/hub) use
MAC addresses. e.g. 01:23:45:67:89:0a

4) Computers don't need to use host or domain names, it's just more
easier to remember a domain name instead of IP address. Also google
might have several IP address, and which one you get might depend upon
factors like your location or device type.

5) The only time you can't use host or domain names is when your
configuring networking.

6) DHCP (Dynamic Host Configuration Protocol) basically automatically
maps your MAC to a IP.

7) Router has say a MAC 01:23:45:67:89:0a an IP 10.1.1.1.

8) You connect your device to the router, in the default setup it will
automatically basically says hello any DHCP servers out their to that
network i'm a new device and my MAC is fe:dc:ba:09:87:65

9) If a DHCP server exists it will say yes, e.g. the router will
answer basically saying i'm a router with MAC 01:23:45:67:89:0a and IP
10.1.1.1, please use IP 10.1.1.64 for your MAC fe:dc:ba:09:87:65. Plus
other routing information along with how long you have that IP address
for. DHCP service is configured with a number of IP address it is
allowed to hand out, say anything from 10.1.1.64 to 10.1.1.127, so 64
IP addresses.

10) Your device wants to talk to google. Your device knows that is
www.google.com currently is IP 74.125.132.104 so sends your request to
that IP.

11) Your device hardware doesn't know what MAC that is for IP
74.125.132.104, so delegates that to your router.

12) Your device hardware then sends that request to your route IP
10.1.1.1 which is knows is MAC 01:23:45:67:89:0a.

13) Your router also also doesn't know what the MAC is for
74.125.132.104, and it is also a public IP address so knows to go off
and delegate that request to your ISP.

14) This continues until a router/device knows the MAC address for the
IP address, and so response.

15) Going past talking to google, if you now put that device to sleep
and start another or wait for 24 hours basically the time it was told
it was allowed the IP address for.

16) The new device also does DHCP and is given say 10.1.1.65 but the
router might be trying to be cleaver and realise that 10.1.1.64 has
not been connected to the networking for a few hours and so deem
10.1.1.64 is avaliable again and hand it back out.

17) The 1st device now starts up and continue to use fe:dc:ba:09:87:65
and 10.1.1.64, it will basically tell the router to keep sending
traffic for 10.1.1.64 to fe:dc:ba:09:87:65. At which point the router
says it's has two MAC address trying to claim traffic for the same IP.

18) Depending upon your router and devices what happens next will be different

I realise those above steps have technical detail and in some places
low level details and in others I've summaries what happens. But
hopefully it briefly explains what happens and what is involves and
anyone wanting more details can search for more details.

In summary beware if sending anything to a domain name address
starting http:// that is finance or personal. As looking back at step
14 your trusting a networking device configured by a person who could
have incorrectly configured by mistake or on purpose.

https:// is safer but their are still potential concerns, especially
on shared computers or untrusted computers.

On another note the following website does not exist
http://www.nonexistingdomainnamerandomtext.com/ if you try to visit it
should should get and error page which your browser decides. If you
get any other page then that is probably your ISP inserting itself in
between steps 1 and 2, where your browser tries to covert a domain
name to an IP address.

John

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[nick_p...@mac.com]

This is the non-technical reply but with the benefit of coming from an Apple Genius the other day.
I have had exactly the same problem recently but it has never really bothered me but (MacBook Air and an iMac) when I was in an Apple store 3 weeks ago I asked about it and I was told that if there is a cordless device like a telephone near the router that can sometimes interfere with the router and cause this. Solution keep other cordless devices away (at least 6ft is what he said) from router. I could only move mine an extra 2 ft away but haven't had the problem since.
By the way thanks John for that very comprehensive reply - almost too technical!!
Nick

[itsa...@mac.com]

Nick and John - thanks very much for your informative replies, especially John for the amazing behind the scenes details. I'll file those for future reference ...

My new iPod touch (yey!) had been added to the network that day AND there is a cordless phone and base station quite close to where the MacBook Air was woken up so it may be those two in combination!

Much appreciated.

Stephen

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