When HTML (HyperText Markup Language) was designed, there was no explicit mechanism in the design to keep track of in software, as this carried additional logistical and network overhead.
Most content management systems include features to track , provided the external site in sends notification to the target site. Most wiki systems include the capability of determining what pages link internally to any given page, but do not track external links to any given page.
Most commercial search engines provide a mechanism to determine the number of backlinks they have recorded to a particular web page. For example, Google can be searched to find the number of pages on the Web pointing to Google only shows a small fraction of the number of pointing to a site. It credits many more backlinks than it shows for each website
Other mechanisms have been developed to track between disparate webpages controlled by organizations that aren't associated with each other. The most notable example of this is TrackBacks between blogs.