Because the evolution of internet technologies has outpaced that of some filtering companies, the number of web sites that are being blocked inadvertently is increasing exponentially, according to the author. What’s worse, few people realize the true reason this is happening.

Web sites can be blocked by URL (such as http://www.cnn.com), IP number (such as http://207.25.71.25), or a combination of the two. The URL is the address you type into a web browser, while the IP number is what a URL is translated into so the computer can access the requested web site’s server. How a filtering company chooses to employ these two methods makes a big difference, thanks in part to something called IP-Independent Virtual Hosting.

Filtering by URL would seem to be the most effective method, but this creates a huge list that is time-consuming and expensive for a company to maintain. To avoid huge lists, most companies use a single IP number to block hundreds of sites. If a server hosts hundreds of adult web sites, for example, a filtering company can use a single IP number to block all of these sites.

Of course, filtering by IP number does not take into account that valid sites can exist on the same servers as adult sites. Therefore, large numbers of valid sites could be excluded along with prohibited web sites. With the rise in IP-Independent Virtual Hosting–a technology that lets internet service providers represent thousands of web sites through a single IP number–the problem of inadvertently blocked sites has grown exponentially, the author says.

To illustrate, the author cites the example of http://oii.org, a web site he’s affiliated with. It was blocked by a well-known (but unnamed) filter because it’s hosted by a server that also hosts adults web sites, and the company uses an IP number to block these inappropriate sites. The author estimates there are 10 legitimate sites blocked on this server for every adult site that’s blocked.

The extent to which companies rely on one technique over the other (blocking by URL or IP number), and whether or not these companies take virtual hosting into consideration, has a big impact on the number of web sites their software filters erroneously. When choosing a filter, ask the company how it handles IP-Independent Virtual Hosting and whether it makes a serious attempt to avoid blocking innocuous sites if its uses IP numbers.