Web filtering is the ability to control web access by sites which are grouped into categories for ease of administration.
Depending on the type of filtering solution there will typically be around 50 to 100+ web categories, the difference in the categorisation tends to come down to the type of technology utilised.
Some solutions scan page content as it is downloaded and use rules to calssify sites into the appropriate categories, whilst this can prove effective for previously unrated sites this can produce unpredictable results due to changing website content.
We find the most accurate classification comes from human review, placing a site into an appropriate category based on content seen at the review time this allows for much more granular review and classification, in addition these URL lookups can be performed in miliseconds as the content is requested and if necessary blocked prior to download reducing bandwidth consumption.
By far the best solution to web filtering is to utilise a URL Database to carry out the initial check for speed backed up by a dynamic classification system for any uncategorised web sites.