Servers deliver a HTTP 404 Not Found header response code when a requested resource was not found. An example of how a server would respond if a file was not found can be seen below:
HTTP 404 Not Found
This Response code would often be send together with a message body, containing a custom error page describing to the user what went wrong. Custom error pages can include a search field, or suggestions to what the user might be looking for.
Problems with 404 errors
Its important that the correct header status code is delivered, along with the error page itself – as there is no reliable alternative method in the HTML, to make search engines understand that they encountered a 404. So called soft 404 errors will not work.
A custom error page must be larger than 512 bytes, to avoid problems with certain browsers and plugins, which will otherwise replace the error page with their own.