Understanding HTTP Status Codes

The five classes

• 1xx — Informational: the request was received, processing continues (rarely seen directly).
• 2xx — Success: the request worked.
• 3xx — Redirection: further action (usually following a new URL) is needed.
• 4xx — Client error: the request was wrong (bad URL, no permission, bad input).
• 5xx — Server error: the request was fine but the server failed to fulfil it.

The ones you'll actually see

• 200 OK — success, the standard healthy response.
• 301 Moved Permanently — the URL has permanently changed; passes SEO value to the new one.
• 302 / 307 Found / Temporary Redirect — a temporary redirect; the original URL stays canonical.
• 304 Not Modified — the cached copy is still valid; nothing re-sent.
• 400 Bad Request — the server couldn't parse the request.
• 401 Unauthorized — authentication required or failed.
• 403 Forbidden — authenticated but not allowed.
• 404 Not Found — the resource doesn't exist.
• 429 Too Many Requests — you're being rate-limited.
• 500 Internal Server Error — a generic server-side failure.
• 502 Bad Gateway — an upstream server returned an invalid response.
• 503 Service Unavailable — the server is overloaded or down for maintenance.
• 504 Gateway Timeout — an upstream server didn't respond in time.

4xx vs 5xx: whose fault is it?

The dividing line is responsibility. A 4xx means the client sent something the server can't or won't handle — fix the request, the URL, or the credentials. A 5xx means the request was valid but the server broke — the fix is on the server or its upstreams, and there's usually nothing the client can do but retry.

What to do with them

• Seeing 200 but the page looks wrong → the status is fine; the problem is in the page content or client rendering.
• 301/302 chains → each hop adds latency; collapse long redirect chains.
• Sudden 5xx spike → check server logs, backend health, and recent deploys.
• Unexpected 403/401 → check auth config, permissions, and any WAF/firewall rules.