HTTP Status Code Reference

HTTP status codes are three-digit numbers returned by a web server in response to a client's request. They indicate whether the request was successful, redirected, resulted in an error, or requires further action. They are grouped into five classes: 1xx (Informational), 2xx (Success), 3xx (Redirection), 4xx (Client Error), and 5xx (Server Error).

200 OK is the most common status code, indicating a successful request. Other very common codes include 301 (Moved Permanently), 304 (Not Modified), 404 (Not Found), and 500 (Internal Server Error). These cover the majority of everyday web traffic responses.

401 Unauthorized means the request lacks valid authentication credentials — the user needs to log in. 403 Forbidden means the server understood the request and the user may be authenticated, but the user does not have permission to access the requested resource.

Use 301 (Moved Permanently) when a resource has permanently moved to a new URL — search engines will update their index. Use 302 (Found) for temporary redirects where the original URL should still be used in the future. 301 passes link equity for SEO, while 302 does not.

5xx status codes indicate server-side errors — the server failed to fulfill a valid request. Common ones are 500 (Internal Server Error, a generic catch-all), 502 (Bad Gateway, upstream server error), 503 (Service Unavailable, server overloaded or in maintenance), and 504 (Gateway Timeout, upstream server didn't respond in time).