HTTP Status Code Reference

HTTP status codes are divided into five classes: 1xx (Informational) means the request was received and the process is continuing; 2xx (Success) indicates the action was successfully received, understood, and accepted — the most common being 200 OK; 3xx (Redirection) means further action needs to be taken by the user agent to fulfill the request, like 301 and 302; 4xx (Client Error) indicates the request contains bad syntax or cannot be fulfilled, with 404 Not Found being the most well-known; and 5xx (Server Error) means the server failed to fulfill an apparent valid request, such as 500 Internal Server Error.

401 Unauthorized means the request lacks valid authentication credentials — the client must authenticate itself to get the requested response. The server challenges the client to provide authentication (e.g., via a login page or API token). 403 Forbidden means the server understood the request but refuses to authorize it — the client's identity is known but they don't have permission to access the resource. Unlike 401, re-authenticating won't help with 403 because the issue is authorization, not authentication.

There are many HTTP status codes defined across various RFC documents. The IANA (Internet Assigned Numbers Authority) maintains the official registry with over 60 standardized codes ranging from 100 Continue to 511 Network Authentication Required. However, only about 30-40 are commonly encountered in everyday web development. This reference covers all standard codes including specialized ones like 418 I'm a Teapot (an April Fools' RFC), 429 Too Many Requests (rate limiting), and 503 Service Unavailable (maintenance mode).

304 Not Modified is a redirect code that tells the client the requested resource has not changed since the last request. Instead of sending the full response body again, the server sends this code to instruct the client to use its cached version. This significantly reduces bandwidth usage and improves page load times, especially for large static assets like images, CSS files, and JavaScript bundles. It's a fundamental part of HTTP conditional request caching and is essential for website performance optimization.

5xx errors indicate server-side failures — the client's request was valid but the server couldn't fulfill it. Common handling strategies include: implementing retry logic with exponential backoff for temporary issues, displaying user-friendly error messages instead of raw error codes, logging detailed error information on the server side for debugging, setting up monitoring and alerting to catch 5xx spikes early, and using load balancers with health checks to route traffic away from unhealthy servers. For your own APIs, ensure you return appropriate 5xx codes with clear error details in the response body.