HTTP Status Codes

HTTP status code classes

HTTP status codes are three-digit numbers returned by a server in response to a client’s request. They are grouped into five classes based on the first digit. 1xx (Informational) codes indicate that the request was received and the process is continuing — these are rarely seen by end users. 2xx (Success) codes confirm that the request was successfully received, understood, and accepted. 3xx (Redirection) codes tell the client that further action is needed to complete the request, usually following a different URL.

4xx (Client Error) codes indicate that the request contains an error on the client’s side — a typo in the URL, missing authentication, or a malformed request body. 5xx (Server Error) codes signal that the server failed to fulfil a valid request due to an internal problem. Understanding which class an error falls into is the first step in debugging: 4xx means fix the client, 5xx means investigate the server.

Most common HTTP status codes

200 OK is the standard success response. 301 Moved Permanently and 302 Found are the two main redirect codes. 304 Not Modified is the caching response that saves bandwidth. 400 Bad Request is the catch-all for client-side errors. 401 Unauthorized means you need to authenticate. 403 Forbidden means you authenticated but lack permission. 404 Not Found is the most famous error code — the resource doesn’t exist at that URL.

On the server side, 500 Internal Server Error is the generic “something went wrong” response. 502 Bad Gateway means a proxy received a bad response from the origin server. 503 Service Unavailable means the server is temporarily overloaded or in maintenance. Each of these codes tells you where to look when debugging — understanding them saves hours of investigation.