There are a number of approaches using software that can be used by parents and schools and libraries to restrict undesirable materials. Some are based on maintaining a list of sites that should be blocked on the basis of their content. Since such sites come and go very rapidly, this approach requires frequent updates to the software to be effective. Some software uses a combination of these techniques. The criteria used to block sites varies widely from one software package to another.
Some packages are very restrictive, and have fallen under criticism for blocking too much.
Others are designed to give the parent or administrator maximum control of what is blocked. The table that follows lists the major software packages available for access control, the platforms they run on, and links to their web sites. If you feel a need for such software, examine each of them, and select what best meets your needs. Reviews of most of these packages can be found at ZDNet .