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Would a Netgear range extender, for example WN2000RPTv2, create a separate network? I wasn't able to tell by looking at the user manual online. How do these things generally work? Does it just, as the name implies, amplify/extend the same network, or is it possible to configure it to function as a router (with its own private subnet, NAT, DHCP, etc.), treating the main network (the one being extended) as a router normally treats the outside world?

So...if I was using an extender to extend a freely available, widely available unprotected wireless network, would the devices connected via the extender be accessible from other devices on the main network that are not connected via the extender? Is there any difference between being attached to the main network, and being attached via the extender? Is the sole purpose of configuring the wireless "security" settings on the extender just to prevent others from using the extender, or is there indeed a private network being created just for the extender clients?

In other words, is there any security difference on a given PC if it connects via an extender vs. just connects directly to the original network?

Thanks!

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These are fake repeaters. Real repeaters require WDS to be configured at the access point. They do a form of NAT that impersonates their clients to the access point. This means seamless roaming is not possible.

The WiFi specification prohibits access points from transmitting data that is not being sent to one of their clients (unless WDS is enabled in the access point). The extender's clients are not clients of the access point. This means the extender has to 'trick' the access point into broadcasting their traffic. It does this by changing their hardware address to the be the extender's hardware address, making the access point think the traffic is to one of its clients.

To answer your questions:

So...if I was using an extender to extend a freely available, widely available unprotected wireless network, would the devices connected via the extender be accessible from other devices on the main network that are not connected via the extender?

It's not clear what you're asking. Accessible how? If you mean as if they were on the same Ethernet network, no. If you mean as if they were on the same IP network, yes, so long as you are using the IP addresses assigned to the subnet and not expecting full bridging.

Is there any difference between being attached to the main network, and being attached via the extender?

Yes. When you're connected to the extender, the extender has to impersonate you to the access point. This means your hardware address will be seen as the extender's hardware address on the original network and your own hardware address on the extender's network. IP doesn't care, but some protocols might.

Is the sole purpose of configuring the wireless "security" settings on the extender just to prevent others from using the extender, or is there indeed a private network being created just for the extender clients?

It is to prevent others from using the extender and allowing the extender to talk to the main network. Most extenders only have one radio, so both networks have to be on the same channel. Generally, matching the SSID and encryption key/mode is a bad idea because seamless roaming won't work and if you match the SSID and encryption information, clients will try to roam.

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Thanks for your response. Interesting. Is this form of "impersonation" as secure as using a router? It sounds like you could not, for example, ping a device behind the extender (could you)? I know someone who is using an extender to extend the free network provided by their living place, in hopes that this is providing security from the main network (likely using whatever the default extender settings are). Seamless roaming is not an objective in this case. Is this a reasonable expectation? Are there forms of attack that he is vulnerable to by using an extender rather than a regular router? –  Josh Apr 24 '13 at 3:43
    
@Josh: You can ping a device behind the extender as ping is an IP thing. No security is provided -- the extender does its best to make everything "just work", it's not trying to stop anything. To answer your last question, the answer is that your question is based on a false premise. A "regular router" doesn't provide any security anyway, except by accident. That it does is a myth. If you want security, you need a firewall. If by "regular router" you mean one with a firewall, then he loses the security the firewall would provide. –  David Schwartz Apr 24 '13 at 15:37
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