When tracking the amount of traffic on a Web site, it refers to a person who visits a Web site more than once within a specified period of time. Software that tracks and counts Web site traffic can distinguish between visitors who only visit the site once and unique visitors who return to the site. Different from a site's hits or page views -- which are measured by the number of files that are requested from a site -- unique visitors are measured according to their unique IP addresses, which are like online fingerprints, and unique visitors are counted only once no matter how many times they visit the site. There are some ISPs that use Dynamic Host Configuration Protocol, such as AOL and cable modem providers, which use different IPs for every file requested, making one visitor look like many. In this case, a single IP address does not indicate a unique visitor.