Understanding HTTP Status Codes: A Comprehensive Guide

HTTP status codes are three-digit numbers returned by a server in response to a client's request made to the server. These codes provide valuable information about the status of the request, helping developers and webmasters troubleshoot issues and enhance the overall user experience. In this comprehensive guide, we'll explore the various classes of HTTP status codes and their meanings.

1. Informational Codes (100-199)

- 100 Continue: The server has received the request headers and the client should proceed with the request.

- 101 Switching Protocols: The requester has asked the server to switch protocols.

- 102 Processing (WebDAV; RFC 2518): A WebDAV request may contain many sub-requests involving file operations, requiring a long time to complete the request. This code indicates that the server has received and is processing the request, but no response is available yet. This prevents the client from timing out and assuming the request was lost. The status code is deprecated.

- 103 Early Hints (RFC 8297): Used to return some response headers before final HTTP message.

2. Success Codes (200-299)

- 200 OK: The request was successful, and the server has returned the requested data.

- 201 Created: The request was successful, and a new resource was created as a result.

- 203 Non-Authoritative Information (since HTTP/1.1): The server is a transforming proxy (e.g. a Web accelerator) that received a 200 OK from its origin, but is returning a modified version of the origin's response.

- 204 No Content: The server successfully processed the request but there is no content to send in the response.

- 205 Reset Content: The server successfully processed the request, asks that the requester reset its document view, and is not returning any content.

- 206 Partial Content: The server is delivering only part of the resource (byte serving) due to a range header sent by the client. The range header is used by HTTP clients to enable resuming of interrupted downloads, or split a download into multiple simultaneous streams.

- 207 Multi-Status (WebDAV; RFC 4918): The message body that follows is by default an XML message and can contain a number of separate response codes, depending on how many sub-requests were made.

- 208 Already Reported (WebDAV; RFC 5842): The members of a DAV binding have already been enumerated in a preceding part of the (multistatus) response, and are not being included again.

- 226 IM Used (RFC 3229): The server has fulfilled a request for the resource, and the response is a representation of the result of one or more instance-manipulations applied to the current instance.

3. Redirection Codes (300-399)

- 300 Multiple Choices: The requested resource corresponds to any one of a set of representations.

- 301 Moved Permanently: The requested resource has been permanently moved to a new location.

- 302 Found (or Moved Temporarily): The requested resource has been temporarily moved to a different location.

- 303 See Other (since HTTP/1.1): The response to the request can be found under another URI using the GET method. When received in response to a POST (or PUT/DELETE), the client should presume that the server has received the data and should issue a new GET request to the given URI.

- 304 Not Modified: The client's cached copy is still valid, and there is no need to send a new request.

- 305 Use Proxy (since HTTP/1.1): The requested resource is available only through a proxy, the address for which is provided in the response. For security reasons, many HTTP clients (such as Mozilla Firefox and Internet Explorer) do not obey this status code.

- 306 Switch Proxy: No longer used. Originally meant "Subsequent requests should use the specified proxy."

- 307 Temporary Redirect (since HTTP/1.1): In this case, the request should be repeated with another URI; however, future requests should still use the original URI. In contrast to how 302 was historically implemented, the request method is not allowed to be changed when reissuing the original request. For example, a POST request should be repeated using another POST request.

- 308 Permanent Redirect: This and all future requests should be directed to the given URI. 308 parallel the behaviour of 301, but does not allow the HTTP method to change. So, for example, submitting a form to a permanently redirected resource may continue smoothly.

4. Client Error Codes (400-499)

- 400 Bad Request: The server cannot understand the request due to a client error.

- 401 Unauthorized: Authentication is required, and the provided credentials are not sufficient.

- 402 Payment Required: Reserved for future use. The original intention was that this code might be used as part of some form of digital cash or micropayment scheme, as proposed, for example, by GNU Taler, but that has not yet happened, and this code is not widely used. Google Developers API uses this status if a particular developer has exceeded the daily limit on requests. Sipgate uses this code if an account does not have sufficient funds to start a call. Shopify uses this code when the store has not paid their fees and is temporarily disabled. Stripe uses this code for failed payments where parameters were correct, for example blocked fraudulent payments.

- 403 Forbidden: The server understood the request but refuses to authorize it.

- 404 Not Found: The requested resource could not be found on the server.

- 405 Method Not Allowed: A request method is not supported for the requested resource; for example, a GET request on a form that requires data to be presented via POST, or a PUT request on a read-only resource.

- 406 Not Acceptable: The requested resource is capable of generating only content not acceptable according to the Accept headers sent in the request. See Content negotiation.

- 407 Proxy Authentication Required: The client must first authenticate itself with the proxy.

- 408 Request Timeout: The server timed out waiting for the request. According to HTTP specifications: "The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time."

- 409 Conflict: Indicates that the request could not be processed because of conflict in the current state of the resource, such as an edit conflict between multiple simultaneous updates.

- 410 Gone: Indicates that the resource requested was previously in use but is no longer available and will not be available again. This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource in the future. Clients such as search engines should remove the resource from their indices. Most use cases do not require clients and search engines to purge the resource, and a "404 Not Found" may be used instead.

- 411 Length Required: The request did not specify the length of its content, which is required by the requested resource.

- 412 Precondition Failed: The server does not meet one of the preconditions that the requester put on the request header fields.

- 413 Payload Too Large: The request is larger than the server is willing or able to process. Previously called "Request Entity Too Large".

- 414 URI Too Long: The URI provided was too long for the server to process. Often the result of too much data being encoded as a query-string of a GET request, in which case it should be converted to a POST request. Called "Request-URI Too Long" previously.

- 415 Unsupported Media Type: The request entity has a media type which the server or resource does not support. For example, the client uploads an image as image/svg+xml, but the server requires that images use a different format.

- 416 Range Not Satisfiable: The client has asked for a portion of the file (byte serving), but the server cannot supply that portion. For example, if the client asked for a part of the file that lies beyond the end of the file. Called "Requested Range Not Satisfiable" previously.

- 417 Expectation Failed: The server cannot meet the requirements of the Expect request-header field.

- 418 I'm a teapot (RFC 2324, RFC 7168): This code was defined in 1998 as one of the traditional IETF April Fools' jokes, in RFC 2324, Hyper Text Coffee Pot Control Protocol, and is not expected to be implemented by actual HTTP servers. The RFC specifies this code should be returned by teapots requested to brew coffee. This HTTP status is used as an Easter egg in some websites, such as Google.com's "I'm a teapot" easter egg. Sometimes, this status code is also used as a response to a blocked request, instead of the more appropriate 403 Forbidden.

- 421 Misdirected Request: The request was directed at a server that is not able to produce a response (for example because of connection reuse).

- 422 Unprocessable Content: The request was well-formed (i.e., syntactically correct) but could not be processed.

- 423 Locked (WebDAV; RFC 4918): The resource that is being accessed is locked.

- 424 Failed Dependency (WebDAV; RFC 4918): The request failed because it depended on another request and that request failed (e.g., a PROPPATCH).

- 425 Too Early (RFC 8470): Indicates that the server is unwilling to risk processing a request that might be replayed.

- 426 Upgrade Required: The client should switch to a different protocol such as TLS/1.3, given in the Upgrade header field.

- 428 Precondition Required (RFC 6585): The origin server requires the request to be conditional. Intended to prevent the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict.

- 429 Too Many Requests (RFC 6585): The user has sent too many requests in a given amount of time. Intended for use with rate-limiting schemes.

- 431 Request Header Fields Too Large (RFC 6585): The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large.

- 451 Unavailable For Legal Reasons (RFC 7725): A server operator has received a legal demand to deny access to a resource or to a set of resources that includes the requested resource. The code 451 was chosen as a reference to the novel Fahrenheit 451 (see the Acknowledgements in the RFC).

5. Server Error Codes (500-599)

- 500 Internal Server Error: A generic error message indicating a server failure.

- 501 Not Implemented: The server does not support the functionality required to fulfill the request.

- 502 Bad Gateway: The server was acting as a gateway or proxy and received an invalid response from the upstream server.

- 503 Service Unavailable: The server is not ready to handle the request. Commonly used for maintenance.

- 504 Gateway Timeout: The server, while acting as a gateway or proxy, did not receive a timely response from an upstream server.

- 505 HTTP Version Not Supported: The server does not support the HTTP version used in the request.

- 506 Variant Also Negotiates (RFC 2295): Transparent content negotiation for the request results in a circular reference.

- 507 Insufficient Storage (WebDAV; RFC 4918). The server is unable to store the representation needed to complete the request.

- 508 Loop Detected (WebDAV; RFC 5842): The server detected an infinite loop while processing the request (sent instead of 208 Already Reported).

- 510 Not Extended (RFC 2774): Further extensions to the request are required for the server to fulfil it.

- 511 Network Authentication Required (RFC 6585): The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g., "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot).

Understanding these HTTP status codes is crucial for web developers and administrators to diagnose and address issues efficiently. Whether it's a successful request, a redirection, or an error, these codes convey valuable information about the interaction between clients and servers on the web.

By grasping the meanings behind these codes, developers can streamline the debugging process, ensuring a smoother and more reliable web experience for users.

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