If a link is made from one web page to another, but the web page cannot be displayed, either because the web server is unavailable or the web page has already been deleted, the Hypertext Transfer Protocol( HTTP) generates an error message via the HTTP status code. Error messages can refer to the clients and the servers and their connections.
The error codes are three- digit numbers. The first digit is an identifier so that the user can directly identify whether the error is client-side or server-side. The client-based error codes start with the digits 4xx, the server-based ones with 5xx. The most popular error pages are reported to the HTTP client with HTTP status codes 404 and 500.
Such error pages occur when websites are renamed, when they are provided with new URLs. They can be designed and contain references and links to other web pages, which has a beneficial effect on their indexing.