What is free public wifi?

http://www.wlanbook.com/free-public-wifi-ssid/

Free Public WiFi
The “Free Public WiFi” SSID seems to be everywhere - at airports, on planes, and in hotels. Based on what the SSID spells, it seems to be the perfect solution for those seeking to connect to the Internet because it is “Free”…cost nothing…”Public”…you break no laws connecting to it…”WiFi” …a few clicks and you’ll be surfing in no time.

Viral SSID
In reality, this SSID is just one of many viral SSIDs that exist anywhere people are with laptops (almost always Microsoft Windows based laptops). In almost all cases this SSID is not a real WiFi hotspot access point but someone else’s laptop in Ad-Hoc mode advertising this SSID. Why does this SSID always appear and more importantly is it harmful if you connected to this SSID?

If you connected to the “Free Public WiFi” SSID in the past, don’t worry…in almost all cases your PC is not infected with a real virus and no data was stolen. Just remember to stop connecting to random wireless networks that you can’t verify as being trusted, especially free ones!

http://baike.c114.net/view.asp?id=13739-39331E0A

要了解这个问题，我们首先来讲下windows 的无线网连接机制：当你刷新无线网的时候，windows会扫描2种不同的连接信号，一种是AP（acess point）无线网络接入点，用这个你可以上网。另一种是Ad hoc network，是无线局域网的一种，可以机器点对点的进行连接。这两种的图标是不一样的，AP是个天线，ADHOC是两个小电脑。
现在去看一下，那个信号很强的"Free Public WiFi”是个什么图标？
对，那只是一个虚假的AD HOC连接，这就是为什么你每次连接他都上不了网的原因。那为什么你及时是在地下，甚至飞机上，都能看到信号很强的"Free Public WiFi”呢？
当你看到一个"Free Public WiFi”，并试图连接到他的时候，你的电脑会保留这个SSID，并开始广播，在别人的电脑看来，你就是一个叫"Free Public WiFi”，信号很强劲的ad hoc network。当别人连接你的时候，他的电脑也成了个叫这个"Free Public WiFi”的ad hoc network。我们可以称这为一个伪病毒。
这在2005年的欧美，开始流行的，windows声称会发布一个补丁，但是我们依然看到，几年以后毫不留情的传到了中国。现在可以说全球到处都是，随着无线网的普及，这个还是会继续流传下去。
下面给出一些常见的这种东东的名字： "hpsetup", "Wireless Network", "Free Public WiFi", "RaddissonWAP1"....等等的，还有很多。。
如果遇到两个小电脑的信号，还是不要点了。