What response codes can a web server return?

1xx: Informational

• 100 Continue: The server has received the initial part of the request and is waiting for the continuation.
• 101 Switching Protocols: The client has requested to switch protocols, and the server is willing to do so.

2xx: Successful operations

• 200 OK: Standard success response.
• 201 Created: The resource has been successfully created as a result of the request.
• 204 No Content: The server has successfully processed the request but will not send a response body.

3xx: Redirects

• 301 Moved Permanently: The resource has been permanently moved to a new URI.
• 302 Found (formerly "Moved Temporarily"): The resource is temporarily located at a different URI.
• 304 Not Modified: The resource has not been modified since the last request.

4xx: Client errors

• 400 Bad Request: The server cannot process the request due to a syntax error or invalid parameters.
• 401 Unauthorized: Authentication is required to access the resource.
• 403 Forbidden: The server understood the request but refuses to authorize it.
• 404 Not Found: The server cannot find the requested resource.
• 405 Method Not Allowed: The method used in the request is not allowed for this resource.

5xx: Server errors

• 500 Internal Server Error: Internal server error.
• 502 Bad Gateway: The server, acting as a gateway or proxy, received an invalid response from the upstream server.
• 503 Service Unavailable: The server is temporarily unable to handle the request, usually due to overload or maintenance.
• 504 Gateway Timeout: The server, acting as a gateway, did not receive a timely response from the upstream server.