Comprehensive HTTP status code reference with meanings, examples, and debugging guidance.
The five response classes and how to reason about them.
Understand the five categories of HTTP responses.
1xx Informational
2xx Success
3xx Redirection
4xx Client Error
5xx Server ErrorHTTP responses are grouped into five classes: informational, successful, redirection, client error, and server error responses. That grouping is reflected in current HTTP references.
A quick flow for deciding where a problem lives.
1) Check request method, URL, headers, and body
2) Inspect auth and permissions
3) Follow redirects if needed
4) Check upstream/app logs for 5xx
5) Confirm caching/proxy/CDN behaviorThis is a practical troubleshooting flow. 4xx responses usually point at client-side request issues or access rules, while 5xx responses usually indicate server-side failure.
Inspect the response line and headers only.
curl -I https://example.comUseful when you only need the status code, headers, redirect location, cache headers, or server details.
Use verbose mode to diagnose redirects, TLS, and headers.
curl -v https://example.comVerbose mode is one of the fastest ways to understand why you received a given HTTP status.
curl -L https://example.comWithout `-L`, curl shows the redirect response; with `-L`, it follows the `Location` header.
Typical status codes used by REST-ish APIs.
200 OK
206 Partial Content
304 Not Modified`200` is the common success response for normal reads; `206` is used with range requests; `304` is used with conditional requests.
201 Created
202 Accepted
204 No Content`201` commonly indicates a resource was created, while `202` means the request was accepted for asynchronous processing and may not be completed yet.
The most common auth-related status codes.
401 Unauthorized
403 Forbidden
407 Proxy Authentication Required`401` typically means authentication is required or invalid, while `403` usually means the server understood the request but refuses it. `407` is the proxy-auth equivalent of `401`.
Responses commonly used when a client must back off.
408 Request Timeout
425 Too Early
429 Too Many Requests
503 Service Unavailable`429` is the canonical rate-limit response; `503` is also often used for temporary overload or maintenance.