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I just bought myself an Ethernet-over-power adapter, and plugged it in. It seems to be working great, but now I'm concerned that my neighbours may be able to access my network.

I live in a rowhome complex; is it possible they can access my network? If so, how can I secure it?

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You should be in a completely different phase than your neighbors so there is no chance that they'd be able plug into it from their homes. – MaQleod Sep 1 '11 at 2:32
You don't mention the manufacturer. I have several Trendnet devices and the permit me to set a private "network name" that isn't broadcast. As password of sorts, if you will. If we knew the maker/model of your powerline devices, we could better answer your question. All of that said, @MaQleod is correct: Anyone on "the other side" of your meter shouldn't be able to see your powerline devices, even if they know the names. – BillP3rd Sep 1 '11 at 2:49
My understanding (not based on technical knowledge, mind) is that these devices are limited to communicating on a single circuit. They can't even communicate between different circuits in the same house. Assuming this is the case, I would be surprised if the signal could readily escape past your power meter. – Slartibartfast Sep 1 '11 at 3:02
@BillP3rd: It's a Trendnet.… – mpen Sep 1 '11 at 6:16
@Slartibartfast: One type of such device (HomePlug AV standard) certainly can communicate between multiple circuits in a single house - you can even connect between two buildings that are on the same utility power supply. Noise may reduce bandwidth in some cases, but a bigger limitation is the fuse box / circuit breaker (aka consumer unit in UK). At least in the UK, there are 3 phases so a house 3 doors away might be on the same phase. So using the device's encryption, with a new key, is recommended. – RichVel Sep 1 '11 at 9:48
up vote 4 down vote accepted

Wikipedia is your friend.

Since signals may travel outside the user's residence or business and be eavesdropped on, HomePlug includes the ability to set an encryption password. The HomePlug specification requires that all devices are set to a default out-of-box password — although a common one. Users should change this password. On many new powerline adapters that come as a boxed pair, a unique security key has already been established and the user does not need to change the password, unless using these with existing powerline adapters, or adding new adapters. Some manufacturers supply adapters with security key buttons on them, allowing users to easily set unique security keys by plugging each unit in one at a time and pressing the button on the front (see more detailed instructions that come with the units).

To simplify the process of configuring passwords on a HomePlug network, each device has a built-in master password, chosen at random by the manufacturer and hard-wired into the device, which is used only for setting the encryption passwords. A printed label on the device lists its master password.

The data at either end (Ethernet side) of the HomePlug link is not encrypted (unless an encrypted higher-layer protocol such as TLS or IPsec is being used), only the link between HomePlug devices is encrypted. The HomePlug AV standard uses 128-bit AES, while the older versions use the less secure DES.

A Homeplug FAQ says

Is HomePlug secure?

Yes it is preset with its own 56bit encryption system to protect your data. HomePlug AV adopts a 128bit encryption system.

Don’t forget to check the security options on any wireless devices in the house. If an outsider can access the wireless network, they also gain access to the homeplug-connected computers, and the homeplug security is useless.

Can my neighbour steal my internet connection if he has a HomePlug?

No, the electricity meter acts as block on the signals that are transmitted across the network, so there is no chance of them leaking out on to the public wiring and your neighbours picking them up.

Don’t forget to check the security options on any wireless devices in the house. If an outsider can access the wireless network, they also gain access to the homeplug-connected computers, and the homeplug security is useless.

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"the electricity meter acts as block on the signals": this is not true. I know cases where someone in one flat (not row house) saw Adapters of his neighbour (flat). – Werner Henze May 3 '13 at 10:58
Also, just because the signal doesn't exit the circuit doesn't mean there aren't other ways in which someone could access your traffic using these devices. For instance, if you have a house with outdoor plugs that you don't check often, someone could easily plug a device into your outdoor plug and connect to your network. They don't even really need to see network traffic they may just be able to do a host scan/see shared drives, etc. So you should probably check outdoor outlets once in a while if you use powerline networking. – deltaray Jun 22 '13 at 6:19

If you are really paranoid, then you can setup a VPN between all your hosts. Perhaps flash your router so that it supports OpenVPN, setup every machine to connect to the OpenVPN server, only permit outgoing connections from the OpenVPN virtual interface.

The setup would be pretty complex though, I doubt it would be worth the effort.

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