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These could be "rogue" hotspots trying to sniff passwords or connect to open shared drives on your machine.

Not every hotspot is as friendly as it should be...

My guess is that when this happens, you are connecting to an access point that doesn't have Internet access

The network and the Internet are 2 completely different things. "The network" consists of the local network infrastructure within whatever facility you're in, be it a hotel, coffee shop, airport, etc. For the sake of discussion though let's say you're in a hotel. The hotel is going to have wireless access points located throughout the structure, and it may also have physical network jacks in each room. This all connects into structured wiring within the hotel itself and routes back to a central data center or closet somewhere within the hotel, and connects into servers, switches, routers, and other gear.

This is "the network", or LAN (local area network). This is the hotels private network. This is not the Internet. Just because you can connect to the hotels private network does mean that you're on the Internet.

Also in the hotels data center or closet they will have communications lines coming in from the street. These lines connect into the same equipment that the hotels private network connects into, and voila, provide Internet access. So from your room you connect to the hotels network, and then you travel over the hotels networks to connect to the Internet.

Jason

The router could be using a MAC filter. MAC = Media Access Control #, not MACintosh. It's actually what I use on my own home network since there's a few devices I have which don't support WPA (and WEP's useless) and there's quite a bit of transient traffic with friends' laptops, phones, and other wireless devices wandering through. To do it the other ways, I'd have to give all of them the password to my router, not ideal. You would get a similar type reaction from my router. You'd "connect" but all packets/traffic would be blocked.

If you see a "linksys", that just means someone bought a Linksys router and did minimal configuring (if any) to it. If they're too lazy to set up their own router, all bets are off to its reliability.

That's not entirely accurate. MAC filtering would prevent him from getting an IP address from the DHCP server, and if he was unable to get an IP address than his laptop wouldn't show as being connected to a network.

And like you said, if someone is too "lazy" to configure their router (ie., they're broadcasting the default linksys SSID), than it's unlikely that they've configured MAC filtering

Yes it would. I "connect" all the time (associate to the AP) but don't get an IP address. I end up with an APIPA address (169.254.n.n) and zero connectivity, but Windows is blissfully ignorant of that problem. In Vista you'll usually get the yellow exclamation point on the connection icon on the systray.

We're saying the same thing, sort of. Connecting / associating with an AP is entirely different than being connected to a network.

What I said is accurate - If you connect to an AP with MAC filtering enabled, you will not get an IP address from the DHCP server and therefore you will not be connected to the network. And as you said, Windows would alert you that there is a problem via the yellow exclamation point.

On the other hand, the OP's original question was in reference to how you could be successfully connected to a network but not have Internet access. In this scenario MAC filtering could not be involved, because if it was, your computer (his Mac, in this case) would not have network connectivity and would alert you that you have connectivity issues.

Yes I admit that we're probably getting too far down the technical track for this thread, and I'm also going off the assumption that the OP would haven mentioned any errors or warnings that he was seeing. Since he eluded to everything appearing normal on his laptop, I am jumping to the conclusion that MAC filtering can be safely ruled out.