Others use an on-site or proxy server at each location. When someone tries to access a site, a short query asks the central computer if the site is appropriate. The server compares the address of the requested page to a list of pre-screened addresses. If the site is appropriate, the server retrieves the address. If a site is blocked, the user will generally get a blank screen with an error message indicating the site has been blocked.
Companies often download updated lists on a daily or weekly basis.
Many companies offer filtering as part of larger security and Internet management software packages. To keep library patrons from squatting on a computer or to help students stay on task, officials can plug in software that turns off the computer after a set amount of time. Monitoring and tracking software, such as Cyber Sentinel by Security Software Systems Inc., allows network administrators to capture screen views and log addresses users visited on the Internet. The software can be combined with another product that displays and forces a user to accept an organization s acceptable use policy before using a specific application. N2H2 Inc. s Bess Filtering Service, named after the inventor s dog, and other programs include built-in caches of popular, approved sites.