Can you list the most common error codes and states that can occur on a server?

The most common error codes and server statuses encountered in web development and testing:

• 1xx (Informational): 100 Continue, 101 Switching Protocols — rarely used directly.
• 2xx (Success): 200 OK, 201 Created — request successfully processed.
• 3xx (Redirection): 301 Moved Permanently, 302 Found — resource has been moved.
• 4xx (Client errors):
  • 400 Bad Request — invalid request.
  • 401 Unauthorized — authentication required.
  • 403 Forbidden — access is forbidden.
  • 404 Not Found — resource not found.
  • 408 Request Timeout — timeout expired.
• 5xx (Server errors):
  • 500 Internal Server Error — internal server error.
  • 502 Bad Gateway — bad response from upstream server.
  • 503 Service Unavailable — service unavailable (e.g., due to overload).
  • 504 Gateway Timeout — gateway timeout.

These codes help diagnose problems and correctly handle server responses in automated testing and monitoring.