How To Understand Http Status Codes
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The five classes
HTTP status codes are the three-digit numbers every request comes back with. The first digit groups them into five classes, and once you know the class you know roughly what happened: 1xx still working, 2xx all good, 3xx look elsewhere, 4xx you asked wrong, 5xx we blew up. Underneath the simple framing lies RFC 9110 (the current HTTP semantics spec, published June 2022) plus about a dozen auxiliary RFCs that add codes for WebDAV, rate limiting, and legal constraints. Picking the right code matters: it drives cache behavior, retries, search-engine indexing, and browser UI like the default error page. This guide covers all five classes with the codes that actually get used in production, the common retry and indexing implications, the difference between 301 and 302, when 503 is correct and when it is lazy, and the codes people reach for that they should not.
1xx informational
The first digit of the status code determines the class:
2xx success
Idempotent methods (GET, PUT, DELETE, HEAD, OPTIONS) are safe to retry. POST generally is not — retrying after a network failure might create the resource twice. Use an idempotency key header on write endpoints for safe retries.
3xx redirection
Googlebot and other crawlers key heavily off status codes.