I just returned from a week-long trip during which I made frequent use of both free and paid Wi-Fi hotspots. One thing I noticed in my travels was that in virtually every venue where I sought wireless access, at least one network had an SSID of "Free Public Wi-Fi" or a similar name. I came across such networks in two airports, a convention center, a hotel lobby and a Starbucks.

I had no idea free Wi-Fi was so ubiquitous  problem is, it's not. On closer inspection I realized that in each case the network purporting to be offering free Wi-Fi wasn't an infrastructure mode network, but rather was one running in ad-hoc mode, which meant that in all likelihood it was there to lure unsuspecting users into connecting to it for some nefarious purpose.

For the unfamiliar, ad-hoc mode is a lesser-known and relatively little-used Wi-Fi capability that enables two or more wireless clients to set up a peer network without an access point. This differs from the far more common infrastructure mode, in which clients connect to an access point instead of directly to each other. Ad-hoc networks are generally used for specific and often temporary purposes (e.g., to directly transfer a file or play a LAN-based game), but seldom are they used to provide Internet access to other computers.

Knowing the Difference

So how can you tell an ad-hoc network from an infrastructure one? Pay close attention to the icons displayed when selecting from a list of available networks. In XP, for example, an infrastructure mode network shows an antenna icon, while the icon for an ad-hoc network is a pair of laptops. In Vista, an infrastructure network's icon resembles a pair of linked computers; for an ad-hoc network, it's three computers arranged like a triangle.

Once you know better than to connect to an ad-hoc network, you may still need to make sure your system doesn't automatically do it for you. Windows Vista will never connect to an ad-hoc network automatically; XP might, depending on how it's configured. To prevent this from happening:

Click the wireless networking icon in the system tray and select View Available Wireless Networks Under Related Tasks, click Change the Order of Preferred Networks Click the Advanced button and select Access Point (Infrastructure) Networks Only To make sure Windows connects only to specified networks, irrespective of type, make sure the box next to Automatically Connect to Non-preferred Networks is clear.

Lest I paint with too broad a brush, I'm not suggesting an ad-hoc wireless network can't possibly be a legitimate hotspot or a source of Internet access under any circumstances. Generally speaking, if you come across an ad-hoc network in a public place  especially one with an alluring name that contains the word "free"  you should definitely be wary, and preferably avoid it altogether.

Should you mistakenly connect to a questionable ad-hoc network, you'll probably realize it soon enough when you don't get the Internet access you were expecting. Nevertheless, a momentary connection may be all someone needs to scan your system for personal information or infect it with some kind of malware.

And even if someone is delivering the Net over an ad-hoc network, it doesn't mean the person's intentions are honorable, since it's a simple matter to provide Internet access by bridging an ad-hoc network with a cellular or other Wi-Fi connection. In fact, this is even more dangerous since it gives someone the opportunity to capture all of your network traffic.

Of course, it's important to remember that even bona-fide hotspots are public networks and using them entails risk. Therefore, regardless of what kind of wireless network you connect to, it's always a good idea to use a firewall that switches to a more secure mode whenever you're on anything other than your home network.

Adapted from PracticallyNetworked.com, part of the EarthWeb.com Network.