Dead link

A dead link or broken link is a link on the world wide web that points to a missing or incorrect webpage or server. Dead links are commonplace on the Internet, but they are considered to be unprofessional. Link rot is the process by which links on a website gradually become dead as time goes on. A study of science education sites suggests that the half life of a collection of links is 5 years

A dead or broken link may have some of the following symptoms:

• If the broken link points to a functioning web server but the page it links to is missing or has been moved, the user sees a 404 error.
• Alternatively, if the page was deliberately removed by the webmaster, the user might see an explanation.
• If the link returns a DNS error, then no web server is registered with that domain name, suggesting the web site is gone, or has switched names.
• When a website's domain registration expires, the domain name is sometimes re-registered by others in order to serve ads to people looking for the old site. Additionally, some groups try to make use of the PageRank accumulated by being the target of the dead links.

In some cases, a dead or broken link points to a page that is by definition consistently changing. This is frequently the case with weblogs (commonly blogs), where a website will link to the front page of the blog. Once more posts have been made to the blog, the post the link refers to can no longer be found on the blog's front page. A permalink is a link that avoid this, because it is an absolute link to the blog post, as opposed to a link to the page the post was featured on.

Dead links persist because people creating or updating a webpage sometimes copy outdated content from other sites without making sure the links work, or because website admins don't recheck their older content to insure that the links still work. There are free tools that use web crawlers to recursively check all the links on a website and insure that they still exist.