What does HTTP status code 404 mean?

404 Not Found means the server cannot locate the requested resource. The URL may be wrong, the page may have been deleted, or it was never created. After repeated 404 responses, Google removes the page from its index.

What is the difference between 401 and 403?

401 Unauthorized means the request requires authentication — the client is not logged in or the token is missing or expired. 403 Forbidden means the client is authenticated but the account lacks permission to access the resource.

What does a 5xx error mean?

5xx codes are server-side failures. The client's request was valid, but the server failed to fulfil it. Check server error logs immediately when debugging 5xx errors. A high rate of 5xx errors signals infrastructure problems.

What is the difference between 301 and 302 redirects for SEO?

301 Moved Permanently tells search engines the URL has changed permanently — PageRank and link equity transfer to the new URL. 302 Found is temporary — search engines continue indexing the original URL. Always use 301 for permanent migrations.

Why should I use 410 instead of 404 for deleted pages?

410 Gone explicitly tells search engines the resource is permanently deleted and should not be crawled again. Google de-indexes 410 pages faster than 404 pages. Use 410 when you intentionally delete content and do not plan to restore it.

What causes a 502 Bad Gateway error?

A 502 means an upstream server (load balancer, reverse proxy, CDN) received an invalid or no response from the origin server. Common causes: origin server crashed, deployment in progress, or a timeout on the upstream connection. Check origin server health and logs.

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HTTP Status Codes

Complete reference for all HTTP status codes — 1xx, 2xx, 3xx, 4xx and 5xx — with plain-English explanations. Free.

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HTTP Status Code Categories

Range	Category	Who is responsible
100–199	Informational	Server acknowledging the request is in progress
200–299	Success	Request succeeded
300–399	Redirection	Client must take additional action (follow redirect)
400–499	Client Error	The request was wrong — client's fault
500–599	Server Error	The request was valid but the server failed — server's fault

Most Common HTTP Status Codes

200 OK — The request succeeded. Standard response for a successful GET, POST, PUT or DELETE.

201 Created — A new resource was created. Returned by POST endpoints after creating a database record.

204 No Content — Successful but no response body. Common for DELETE and PUT operations.

301 Moved Permanently — The resource has a new permanent URL. Search engines transfer link equity to the new URL. Use for permanent migrations.

302 Found — Temporary redirect. Search engines keep indexing the original URL. Use for A/B tests and temporary maintenance pages.

400 Bad Request — The server could not understand the request. Check headers, body format and query parameters.

401 Unauthorized — Authentication is required or has failed. Check your API token, session cookie or login credentials.

403 Forbidden — Authenticated but not authorised. The account lacks the required role or permission.

404 Not Found — The resource does not exist at this URL. Check the path, verify the resource exists.

429 Too Many Requests — Rate limit exceeded. Implement exponential backoff and respect the Retry-After header.

500 Internal Server Error — Generic server failure. Check server logs for the root cause.

502 Bad Gateway — An upstream server (load balancer, proxy, CDN) received an invalid response from the origin server.

503 Service Unavailable — Server temporarily overloaded or in planned maintenance. Usually temporary.