404 Not Found
A 404 response is suitable for when a resource was unable to be found at the requested URL.
If a server administrator knows that a resource will no longer be available on a permanent basis then a 410 Gone status code may be preferable.
A 404 is in the 4xx class of status codes which are client error based.
There are just less than 30 other status codes in the 4xx class.
Yes a 404 can impact SEO. Generally speaking websites should avoid internally linking to 404 pages.
301 redirects may be preferable in some circumstances.
If you are using a CMS such as WordPress, usually deleting a page will set it to be a 404 automatically.
If you need to set a 404 manually there are various methods, depending on what type of CMS and/or Server you are running.
Because 404 pages are often seen by visitors it is a good idea to have a custom or 'friendly' 404 page.
A simple message with a search box and link to the homepage is a lot better for users than a default server error page.
The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource or is not willing to disclose that one exists. A 404 status code does not indicate whether this lack of representation is temporary or permanent; the 410 (Gone) status code is preferred over 404 if the origin server knows, presumably through some configurable means, that the condition is likely to be permanent. A 404 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [RFC7234]).
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
Content last checked for accuracy and updated: 1st August 2020, by Colin McDermott