internal URI for a Wikipedia article named Example, which is http://en.wikipedia.org/w/index.php?title=Example, is generated via URL rewriting from the more human-readable external URL, http://en.wikipedia.org/wiki/Example. An entry in a blog with many entries is accessible from the site's front page for only a short time. Visitors who store the URL for a particular entry often find upon their return that the desired content has been replaced by something new. Prominently posting permalinks is a method employed by bloggers to encourage visitors to store a more long-lived URL (the permalink) for reference. Permalinks frequently consist of a string of characters that represent the date and time of posting, and an identifier that denotes the author who initially authored the item or its subject. Crucially, if an item is changed, renamed, or moved within the internal database, its permalink remains unaltered, as it functions as a magic cookie that references an internal database identifier. If an item is deleted altogether, its permalink can frequently not be reused. Permalinks have subsequently been exploited for a number of innovations, including link tracing and link trackback in weblogs, and referring to specific weblog entries in RSS or Atom syndication streams. Comparing with PURL Both permalink and PURL (persistent uniform resource locator) are used as a persistent URL, and redirect to the location of the requested web resource. The main differences in the concepts are about domain name and time scale: PURL uses an independent dedicated domain name, and is intended to last for decades; permalinks usually do not change the URL's domain, and are intended for use on timescales of years. Permalinks and versions Many blogging and content management systems do not support versioning of content, that is, if an entry is updated, a uniquely accessible version is not created. Thus, in the context of these systems, a permalink may refer to different content over time. In the context of systems that support versioning, such as most wikis, a permalink is commonly understood as a link to a specific version. Here, both the link itself and the resource it refers to should not change over time. MediaWiki, the software that runs Wikipedia, supports this type of permanent link. In its current implementation, old versions of specific articles, images, and templates are referenceable by unique unchanging URLs, though current entries may not use old versions of images and templates. Permanent links to specific versions are recommended for citing