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I noticed a new machine on my network -- it's my next-door neighbor's Airport Extreme!

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My network is set up with a WPA2 Personal password, and I don't believe that he's cracking passwords to steal my bandwidth. My suspicion is that it's accidental. I don't see any place in Airport Utility where I can say "don't allow this network to be extended".

I'd like to know

  1. Is it actually possible that his Extreme is extending my network, even without having the password?
  2. Is it possible for me to block it from doing so?
  3. Is there any security flaw here that I should be worried about? Or is this actually just preventing him from being able to connect via wifi (because it's all my network now)?
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    Click on your TC and his Airport Extreme and check IP numbers. Most likely you will find that you have the same ISP and that you are connected to the same router at the ISP and are on the same subnet so the Bonjour information can seep through.
    – Thorbjørn Ravn Andersen
    Apr 10, 2013 at 23:41
  • @ThorbjørnRavnAndersen Good idea. We managed to figure it out, and this was more or less what was happening -- our networks were physically merged out at the street because we were both using IP-over-power.
    – Nathaniel Waisbrot
    Apr 11, 2013 at 0:25

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After the neighbor and I spent some time going over our networks, we figured out the problem: we were both extending our home networks over the house power lines, and apparently the street power for our row houses doesn't provide much separation.

The HomePlug system, which is what both of our adapters conform to, allows you to set up encryption on the network traffic. Not only does this keep things secure, it also keeps other networks from getting tangled. However, both of us had gotten systems that default to no encryption and we'd blithely left them that way. (Mine is a Netgear Powerline AV 500, my neighbor has some other manufacturer's)

As Thorbjørn suggested, the network graph that the Airport utility was showing was just a source of confusion. Once we'd separated the HomePlug devices, everything went back to normal.

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  • Great & interesting network trouble shooting ☺! This is a bug within AirPort utility. It failed to see that your TimeCapsule and your neighbor's AirPort Extreme were connected through Ethernet and not through Wi-Fi.
    – dan
    Jul 18, 2013 at 6:45
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    Just thought I'd clarify - the solid line connecting the two routers indicates that it is a wired connection (HomePlug behaves similarly to Ethernet). A detected wireless connection would show up as a dotted line.
    – Daniel
    Sep 25, 2014 at 17:23

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