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When I browse thru the list of wifi networks in my neighbour, I see a couple of unprotected networks (with no wpa/wpa2 or wep protection). When I try to connect, I cannot. These networks belong to Wifi ISPs and you have to buy their service to be able to connect via their APs.

So how do we call such Wifi APs? They're neither protected nor open.

How do such networks protect unwanted users to connect to them? Is there some host or similar file on user's comp which allows them to connect?

Thanks anyone for taking time to clarify these things to me.

EDIT

You cannot actually even connect to this network. It looks like it's open and I can see it says "unprotected", but yet I cannot connect to it.

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I suppose you mean they bring you to a webpage that ask you to login? See: en.wikipedia.org/wiki/Captive_portal –  Terry Chia Aug 30 '12 at 15:05
    
@TerryChia Actually, you cannot even connect, thou it says "unprotected network" –  clearojne Aug 30 '12 at 15:22
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"When I try to connect, I cannot." "it does not work" is not a proper way to describe a computer issue. What have you tried exactly, and what do you observe exactly. –  curiousguy Aug 30 '12 at 15:40
    
+1 interesting problem –  Daniel Li Aug 30 '12 at 15:47
    
@curiousguy I see new wifi network in win7 taskbar, I click on it and see this network. I try to connect to it (as it appears to be open), but then I receive win7 error that I cannot connect. –  clearojne Aug 31 '12 at 8:04
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1 Answer

up vote 3 down vote accepted

They could be using MAC-based filtering which only permits specific network cards to access them. The AP has to know the MAC address of each network card permitted to access it. That would be a little unusual for ISPs, but certainly could happen.

Such protection is trivially defeated by forging the MAC address of the client. MAC addresses have traditionally been meant to be hard-wired and unique, and just as traditionally been so easily fungible as to defeat any security method based on it. Nonetheless, vendors still include it as an option.

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Well you still have to guess the right user's MAC, right? Does not sound so easy. –  clearojne Aug 30 '12 at 15:20
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@clearojne You don't guess, you capture a valid MAC number over the air, and use it. –  curiousguy Aug 30 '12 at 15:41
    
Surprisingly enough, valid users transmit their MAC with every single packet they send! All you need to do is stick around long enough to watch at least one person connect. –  gowenfawr Aug 30 '12 at 15:42
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It is also worth noting that MAC filtering will not prevent anyone in the area from being able to see any data sent across the wireless network unless it is otherwise protected (for example, by SSL). –  AJ Henderson Aug 30 '12 at 19:36
    
MAC-based filtering takes about 10s to bypass (depending on how fast you can type a ifconfig command) but you need an authorized MAC address and connect when the "legal" client is not connected. Open Wifi network with "free wifi" limited to XX minutes or hours uses MAC address list to record last connections which you can bypass by changing your MAC address. I have a question though, is it illegal to connect to bypass such filters (auth or connection time or any other limitation)? An "attacker" do not make any kind of modification to the AP or any server. –  TecHunter Aug 31 '12 at 12:53
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