Not displaying Web pages with URLs that are known to be used for pornography Web sites. The problem is that no filter is perfect. Filters can cut out Web sites that may be useful, even important. For example, Web sites on breast cancer, on sexual disfunctions, on psychotherapy. The entire Yahoo Internet index site was once excluded by filtering software.
Filters can allow some x-rated Web pages if there is nothing on the specific page to be excluded.
Children sometimes learn to disable filters.