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🌐 HTTP Status Codes – Complete Notes

HTTP status codes are grouped into 5 categories based on their first digit.
Each category has common codes you should know for development and interviews.

🔹 1xx – Informational

  • Request was received and understood.
  • Server is still processing the request.
  • Rarely seen in practice.

Common Codes

  • 100 Continue → Initial part of request received, client should continue.
  • 101 Switching Protocols → Protocol change requested by client is accepted.
  • 102 Processing → Server has received request and is processing, but no response yet.

🔹 2xx – Success

  • Request was successfully received, understood, and processed.

Common Codes

  • 200 OK → Standard success response.
  • 201 Created → Resource successfully created.
  • 202 Accepted → Request accepted for processing, not completed yet.
  • 204 No Content → Request processed successfully, no response body.

🔹 3xx – Redirection

  • Further action needed to complete the request.
  • Often used for URL redirects.

Common Codes

  • 301 Moved Permanently → Resource moved to a new permanent URL.
  • 302 Found → Temporary redirect.
  • 304 Not Modified → Cached version can be used, no need to download again.
  • 307 Temporary Redirect → Similar to 302, but keeps request method (e.g., POST stays POST).
  • 308 Permanent Redirect → Like 301, but keeps request method.

🔹 4xx – Client Error

  • Error occurred on the client’s side.
  • Usually due to bad requests, missing data, or lack of permissions.

Common Codes

  • 400 Bad Request → Invalid syntax or malformed request.
  • 401 Unauthorized → Authentication required (or failed).
  • 403 Forbidden → Client authenticated but not allowed to access resource.
  • 404 Not Found → Resource not found on the server.
  • 405 Method Not Allowed → HTTP method not supported by resource.
  • 408 Request Timeout → Client took too long to send request.
  • 429 Too Many Requests → Client sent too many requests in a given time.

🔹 5xx – Server Error

  • Error occurred on the server’s side while processing request.

Common Codes

  • 500 Internal Server Error → Generic server failure.
  • 501 Not Implemented → Server does not support requested functionality.
  • 502 Bad Gateway → Invalid response from upstream server.
  • 503 Service Unavailable → Server overloaded or down for maintenance.
  • 504 Gateway Timeout → Upstream server did not respond in time.
  • 505 HTTP Version Not Supported → Server doesn’t support requested HTTP version.