HTTP Status Code
Definition
Codes are grouped by their first digit, and the group matters more than the individual number. The 1xx family is informational and rarely seen. The 2xx family means the request succeeded. The 3xx family means the resource lives somewhere else or the copy you already hold is still good. The 4xx family blames the request — a wrong address, missing credentials, a file too large. The 5xx family blames the server, which is to say something on the host's side went wrong or is overloaded. The distinction between 4xx and 5xx is the one that decides who has to fix it. Browsers act on many of these automatically, so a visitor usually only notices the ones that produce an error page.
Why It Matters
The code drives behaviour in machines that never look at the page body. A search crawler treats 301 as permanent and eventually transfers the old address's standing to the new one, while 302 says leave things as they are. It will retry a 503 and come back for a 404, but a 410 tells it the page is deliberately gone and to stop asking. A browser caches a 301 aggressively — sometimes until the cache is cleared — which is why publishing the wrong permanent redirect is far harder to undo than publishing the wrong page. Getting the code right costs nothing at the time and saves a week of confusion later.
How It Works
Roughly a dozen codes cover almost everything a hosting user meets. 200 OK is the normal success. 301 Moved Permanently and 302 Found both send the browser elsewhere through a Location header, permanently and temporarily in turn. 304 Not Modified is the empty reply to a conditional request, saying the copy matching your ETag is still current, and it carries no body at all. 403 Forbidden means the file exists but you may not have it; 404 Not Found means nothing is there; 410 Gone means it was there and was removed on purpose. 413 Content Too Large rejects an upload over the size limit, 429 Too Many Requests throttles a client sending too fast, 500 Internal Server Error covers an unhandled fault and 503 Service Unavailable means temporarily down or overloaded, usually with a Retry-After header. Of the lot, the 301 redirect is the one worth understanding properly, because its effects are permanent by design.
Real-World Example
Someone reorganises a set of PDFs on 99helpers and the old address specs-v1.99helpers.site now returns 301 pointing at specs-v2.99helpers.site, so existing links and their search standing carry over. A file deleted for legal reasons is configured to return 410 rather than 404, so crawlers drop it quickly. During a large re-upload the site briefly answers 503 with Retry-After: 120, which tells crawlers to come back rather than record the site as broken.
Common Mistakes
- ✕Returning 200 with an error page in the body — crawlers index the apology as real content, a habit known as a soft 404
- ✕Using 302 for a move that is permanent, so the old address keeps its rankings and the new one never accumulates any
- ✕Answering 404 for content deliberately removed, when 410 tells crawlers to stop checking within days rather than months
- ✕Serving 503 with no Retry-After header during maintenance, leaving clients to guess when to come back
Related Terms
301 Redirect
A 301 redirect is a response telling the browser that a page has moved permanently and giving the new address. The old URL keeps working for anyone who follows the old link, but everything that matters now lives at the new one.
Custom 404 Page
A custom 404 page is the page a site shows when someone asks for an address that does not exist, written by you rather than left to the host's default. It is returned with a 404 status code.
ETag
A short identifier a server attaches to a file so a cache can later ask whether that exact version is still current. If it is, the answer comes back as a 304 with no body, and nothing is downloaded.
Cache-Control
The HTTP response header that tells browsers and caches whether they may keep a copy of a file, for how long, and who is allowed to. It is the single biggest lever over how fast a site feels on a second visit.
HTTPS Redirect
A rule that answers any request arriving on plain HTTP with an instruction to fetch the same address over an encrypted connection instead. It makes the secure version the only one visitors actually see.
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