The HTTP Status Code System
HTTP status codes are three-digit numbers returned by a server in response to every request. They are grouped into five classes: 1xx (informational), 2xx (success), 3xx (redirection), 4xx (client errors), and 5xx (server errors).
The Most Important 2xx Codes
200 OK is the most common success code. 201 Created is returned when a resource is successfully created (POST requests). 204 No Content is used when an operation succeeds but there is nothing to return (DELETE requests).
Redirection Codes That Matter for SEO
301 Moved Permanently passes link equity to the new URL — use it for permanent redirects. 302 Found is a temporary redirect that does not pass link equity. 307 Temporary Redirect and 308 Permanent Redirect preserve the HTTP method during redirect.
Common 4xx Errors
400 Bad Request means malformed syntax. 401 Unauthorized requires authentication. 403 Forbidden means authenticated but not permitted. 404 Not Found means the resource does not exist. 429 Too Many Requests signals rate limiting.
5xx Server Errors
500 Internal Server Error is a generic catch-all. 502 Bad Gateway means the upstream server returned an invalid response. 503 Service Unavailable means the server is temporarily overloaded or down. 504 Gateway Timeout means the upstream server timed out.
