It is filtered by a mix of keyword & block lists.
Some keyword filtering is done live, but a lot of done by block list.
Some of the keyword is very ineffuctal and often turned off my local admins. EG www.exspertsexchange.com a computer site would be censored for being a p0rn site.(The block list contains the 'keys of heaven', every sleazy naughty, fun & opposing political view site there is) http://www.peacefire.org is the best resource for understanding how blocks work, and what types there are. Dont be suprised if that site is blocked though :|
Very interesting reading about what sites get blocked, and the error rates.
Some classic stories like:
For five months, the makers of WebSENSE blocking software published a daily list of pornographic Web sites that were not blocked by their competitors, allegedly to show that their own product was superior. Of course, imo internet censorship just doesnt work. Peacefire also runs Circumventor sites that skip past most blocks. But be careful, as visiting these sites may be a voilation of your 'acceptable use policy' and if you signed a AUP saying you wouldnt surf for fun and do so, well it's your job not mine.
If you havent got an AUP.. well I guess how are you supposed to know you shouldnt 
I do not suggest you use peacefire, google cache, or any other Circumventors if your work has a policy against it. Admin can veiw everything you do if they choose, and most of the time it is ruled legal for them to watch your web surfing, email etc. |