Somebody who knows for sure will chime in shortly, but I suspect you will not be assigned a good ip address unless you sign in. Without one, you get no traffic.
They probably use some sort of Proxy server that you must go through for network access, and likely have a firewall set to deny any packets that originate from anywhere other than that proxy server.
I’m a little in the dark on the way all of that works as well. I’m interested in learning more, because where I work we are wanting to setup public wireless access and want to use a terms of use page that loads every time someone opens a browser on that network where the user will have to agree to our terms before being allowed access. I’ve seen it done at other sites so I know it can be done.
If any FReepers out there could enlighten me, I’d be most appreciative.
Any wireless router can do it with the right firmware, such as DD-WRT. The router hijacks http requests of unauthenticated users and allows nothing else. Upon opening a web browser, they’re redirected to a web page — one on your own server, or one at a hot spot service. After authentication the redirect goes away and any other Internet traffic is opened to the user (depending on what you allow).
If you have an old computer and a wireless access point, ZoneCD is a Linux distro built just for this, including optional payment. It’s a live CD, no install necessary.