The response or status codes of HTTP directly determine the meaning of the response to an HTTP request. They are always transmitted in the form of the first line of the response (= server response), which consists of a three-digit number and a brief description. The status codes are divided into groups of different meanings after their first digit.

The following list is not exhaustive since every application of HTTP can define its own status codes, as WebDAV does for example(Web-based Distributed Authoring and Versioning).

In the following, the requesting program, eg a browser, is always called “Client”; “Server” always denotes the responding program, eg the server of a website.

There is hardly an Internet user who does not know this message: “Error 404 – this page does not exist” or with a shorter version: “404 – not found.” This is exactly an HTTP status code. Status stands for the reaction of the server to a request. The request is usually to display a specific document—almost always an HTML page. And code stands for a three-digit number. The principle is therefore very simple:

The client (i.e. the browser) sends a request (HTTP request).
The server processes the request and sends a response (HTTP response).
The answer is in the HTTP status code.
After a correct request, the server sends the client a class 2 message, for example, the status code 200 for “OK”. As an Internet user, you will not notice this message – apart from the fact that the requested page is displayed correctly in the browser.

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What are the HTTP Status Codes?

HTTP is short for Hypertext Transfer Protocol. This is the technical standard used for sending and receiving multimedia documents on the Internet. An HTTP status code is sent as a response to the request. The header of the protocol provides a code that contains important parameters that affect, for example, the language or the character set of the document. In addition, there is often information about the client. There is also the Hypertext Transfer Protocol Secure (HTTPS), which creates a bug-proof connection between the user’s browser and the website. FTP stands for File Transfer Protocol. This procedure is used for uploading and downloading files.

The status codes of HTTP messages in the header are divided into five groups. The first digit in a request code denotes the status class.

Status 1: Processing of the request continues. Example: 100 (Continue). With this answer, the user can continue with his extensive request.
Status 2: The processing was successful. Example: 204 (No Content). This status means that the request was successful, but deliberately no document is displayed or content is delivered. However, there is no error.
Status 3: The request is redirected. Example: 305 (Use Proxy). In this case, the requested document (requested resource) can only be reached via a proxy. This proxy is shown in the header as a location.
Status 4: There is an error for which the client is responsible. Example: 413 (Request Entity Too Large). The server cannot process the request because the request is too extensive.
Status 5: It is an error that is the responsibility of the server. Example: 500 (Internal Server Error). An unexpected server error.
The status classes with the numbers 1 to 5 are documented in RFC. RFC is short for Request for Comments. These are documents that describe the basics of network technology and are published by a group in the Internet Society ISOC.

Two Known Bugs for HTTP Status Codes: 404 and 403

Not found. This is probably the best-known status code. The simple reason for this is that a specific HTML page or other resource is not found under the URL entered. Possible reasons for this are:

The user made a typing mistake and asked for a page that does not exist.
The link that the user clicked on is incorrect.
The resource was available but has since been deleted because, for example, it was only used for temporary purposes. The URI is simply out of date.

Every now and then a surfer comes across the message “forbidden” (forbidden). This message appears with error 403, which is one of the better-known codes alongside 404. In this case, the user has requested a page for which special authorizations are required. However, it is usually due to the browser settings of the client or a firewall that the page cannot be called up.

Note: The Google Webmaster Tools provide Internet operators with various options for analyzing crawl errors on their own site. You can check the Googlebots’ Response’s through URL Inspection Tool.

HTTP Status Codes and Google’s Guidelines for Indexing

Website operators often ask themselves what impact HTTP status codes have on SEO (Search Engine Optimization). Google Robots are known to crawl websites on a regular basis. This is also done, for example, to delete content that cannot be accessed from the Google directory. It is therefore important to regularly check HTTP status codes, for example, to check the functioning of the server.

For your own website, it is also negative under SEO aspects if it contains a lot of dead links. The status code “Error 404” on the content requested via a link often speaks for the fact that your own website is also out of date—and this is also seen by Google algorithms.

Importance of a Customized 404 Page

It is especially important for SEO to create your own error pages. Because a poor machine response to the request does not exactly invite a visitor to continue searching on the targeted domain. In order not to lose any traffic, it is advisable to create a separate page for error messages. It should be designed in the design of your own website and, for example, prompt the user to visit the homepage in order to continue searching there. Many error message pages also offer the visitor a search field. Technically, the redirection works (for example to a page with the name fault message.html) with only one line of code:

ErrorDocument 404 / errormessage.html.

This text is written in the .htaccess file, which is stored in the root directory of the server. This is also possible for other error messages.

1xx: Informative HTTP Status Codes

You can see the meaning, definition, and function of 100 Status Codes, in the table below.

Code for HTTP Status Codes	Designation of HTTP Status Code	HTTP version	Description of HTTP Status Code
`100 Continue`	Continue	HTTP 1.1	The client should continue the request. This response is used to inform the client that the request has been accepted and that he can proceed with further data. As a rule, this code is only sent if the client has also sent an Expect: 100-continue header in the request.
`101 Switching Protocols`	Exchange log	HTTP 1.1	If the request `Upgrade`contains a header that is acceptable to the server, this confirms that the protocol has been changed. Immediately after this status code and an empty line, the server begins transmission in the new protocol. An example is a change from HTTP to HTTP over TLS according to RFC 2817.
`102 Processing`	in processing	HTTP 1.1	Used to avoid a timeout while the server is processing a time-consuming request (RFC 2518)

HTTP 100 Status Codes

2xx: Success HTTP Status Codes

You can see the meaning, definition, and function of 200 Status Codes, in the table below.

Code for HTTP Status Codes	Designation of HTTP Status Code	HTTP version	Description of HTTP Status Code
`200 OK`	OK	HTTP 1.1	The request was successful. Depending on the request method, the response contains the requested data.
`201 Created`	Generated	HTTP 1.1	The request resulted in a new resource being successfully created. Their URI can be `Location`communicated in the response header.
`202 Accepted`	Accepted	HTTP 1.1	The request has been accepted but is not yet being processed. It is not possible to receive status information when the processing is actually taking place, for example, to determine success.
`203 Non-Authoritative Information`	Non-binding information	HTTP 1.1	The meta-information in the response header may not be the same as that sent by the origin server. Answer fields can be changed by proxies, for example.
`204 No Content`	No content	HTTP 1.1	The request has been accepted and processed, but there is no content that can be returned. In this case, clients should continue to display the old content.
`205 Reset Content`	Reset content	HTTP 1.1	The request was successful and the client is asked to restore the previously viewed document to its original state. This corresponds, for example, to pressing a reset button `<input type="reset"/>` in an HTML form.
`206 Partial Content`	Partial content	HTTP 1.1	If the request `Range`contains a header, the server replies with this status code that the response is sent back in the desired area. One example is interrupted and resumed downloads.
`207 Multi-Status`	Multi-status	HTTP 1.1	The answer contains an XML document that contains several status codes for independently performed operations.
`208 Already Reported`	already reported	HTTP 1.1	WebDAV RFC 5842 – The members of a WebDAV binding have already been listed and are no longer available in this request.
`226 IM Used`	Instance manipulation used	HTTP 1.1	RFC 3229 – The server has fulfilled a GET request for the resource, the response is a representation of the result of one or more instance manipulations related to the current instance.

HTTP 200 Status Codes

3xx: Redirect HTTP Status Codes

You can see the meaning, definition, and function of 300 Status Codes, in the table below.

Code for HTTP Status Codes	Designation of HTTP Status Code	HTTP version	Description of HTTP Status Code
`300 Multiple Choices`	More options	HTTP 1.1	The requested resource corresponds to a set of possible resources from which the client should choose. If the server has a preferred destination, this should be sent in a `Location`header.
`301 Moved Permanently`	Permanently postponed	HTTP 1.1	A new URI has been permanently assigned to the requested resource. This should be sent in the `Location`header. If the request method was `GET`or `HEAD`, the client can automatically forward to the new URI, otherwise it can only do so after user input.
`302 Found`	Found	HTTP 1.1	The requested resource was found, but currently has a different URI. Nevertheless, the client should continue to request this resource under this URI. Forwarding is to be `301`handled analogously to the status.
`303 See Other`	See elsewhere	HTTP 1.1	The answer can be `GET`fetched under another URI using the method. The purpose is `POST`to be able to seamlessly forward to a selected resource after a request.
`304 Not Modified`	Not changed	HTTP 1.1	The resource has not changed since the last request, the answer contains no data. This is the case, for example, if the client `If-Modified-Since`uses a header to declare that it already has the resource in an older version.
`305 Use Proxy`	Use proxy	HTTP 1.1	The resource must be accessed through the proxy `Location`specified in the header.
`306 (unused)`	(not used)	HTTP 1.1	The status code was `Switch Proxy`reserved for the answer in an earlier version , but is no longer used.
`307 Temporary Redirect`	Temporary diversion	HTTP 1.1	The requested resource was found but currently has a different URI. However, the client should continue to request this resource under this URI. Forwarding is to be 301handled analogously to the status. Since in older browser implementations the 302status was handled in such a way that the forwarding was independent of the query method used, HTTP 1.1 303and introduced 307to give servers the possibility to explicitly control the forwarding. Another problem is URL hijacking, which should be avoided.
`308 Permanent Redirect`	permanent diversion	HTTP 1.1	Introduced experimentally via RFC; the requested resource is now available at the address specified in the “Location” header field, the old address is no longer valid. The browser should follow the same method as for the original request (ie a POST is followed by a POST). This is the main difference to 302/303. ^[1 ]

HTTP 300 Status Codes

4xx: Client Error HTTP Status Codes

You can see the meaning, definition, and function of 400 Status Codes, in the table below.

Code for HTTP Status Codes	Designation of HTTP Status Code	HTTP version	Description of HTTP Status Code
`400 Bad Request`	invalid request	HTTP 1.1	The client sent an incorrect request that the server cannot process due to incorrect syntax.
`401 Unauthorized`	Unauthorized	HTTP 1.1	The request cannot be processed without the authorization. To do this, the server must send a WWW_Authenticate response header that explains how to authenticate. HTTP predefines the two methods “Basic” and “Digest”.
`402 Payment Required`	Payment needed	HTTP 1.1	This status code is reserved for future use. It also means that for further processes, the client should make payment.
`403 Forbidden`	Forbidden	HTTP 1.1	Access is permanently prohibited. Authorization is not recognized and the client should not make the request again.
`404 Not Found`	Not Found	HTTP 1.1	The server could not find the resource requested by the client. The most common cause is a so-called dead link. The `404`status can also be returned if no other status code is applicable or the client is to be rejected deliberately without giving a specific reason.
`405 Method Not Allowed`	Method not allowed	HTTP 1.1	The request method (like GET or POST) is not allowed. This answer can come back, for example, if you use WebDAV methods without the server being able to do this. The server must list permitted methods in an Allow response header.
`406 Not Acceptable`	Unacceptable	HTTP 1.1	The resource cannot be delivered in a form requested by the client. For example, an image with the media type can be `image/gif`requested, but the server only finds one hit for `image/png`.
`407 Proxy Authentication Required`	Proxy authentication required	HTTP 1.1	Similar to the 401 response, this response requires authentication. In this case, however, the client must identify himself as a proxy. To do this, the proxy must provide the answer field `Proxy-Authenticate`.
`408 Request Time-out`	Request timed out	HTTP 1.1	The request was not completed by the client in the time specified by the server. The error can occur, for example, when uploading large files.
`409 Conflict`	conflict	HTTP 1.1	The request cannot be processed because this would result in a conflict with the requested resource. If, for example, a resource is to be changed using the PUT method, but which already has a newer version on the server, the processing would create a conflict.
`410 Gone`	Disappeared	HTTP 1.1	The resource no longer exists, but the server knows no forwarding address. This condition can be assumed to be permanent.
`411 Length Required`	Length needed	HTTP 1.1	The server refuses to process the request without a “content-length” header.
`412 Precondition Failed`	Precondition failed	HTTP 1.1	A condition specified by the client cannot be met.
`413 Request Entity Too Large`	Request entity too large	HTTP 1.1	The content of the request (eg an uploaded file) is too large, the request is rejected.
`414 Request-URI Too Large`	Request URI too long	HTTP 1.1	The request URI is too long. The HTTP standard does not define a fixed upper limit for URI lengths, ie the appearance of this error depends on the server program and its internal setting.
`415 Unsupported Media Type`	Unsupported media type	HTTP 1.1	The requested media or MIME type is not supported.
`416 Requested range not satisfiable`	The inquiry area cannot be fulfilled	HTTP 1.1	The requested section of the resource does not exist or is invalid. This can occur in connection with partial downloads, for example.
`417 Expectation Failed`	Expectation failed	HTTP 1.1	The expectations of the client, expressed in the “Expect” header, cannot be met. If the server is a proxy, this can mean, for example, that it cannot address the next jump address.
`420 Policy Not Fulfilled`	Condition not met	HTTP 1.1	W3C PEP (Working Draft November 21, 1997) suggests this code indicates that a condition has not been met.
`421 Misdirected Request`	Misguided request	HTTP 1.1	The request was sent to a server that is unable to send a response. Introduced in HTTP / 2.
`422 Unprocessable Entity`	cannot be processed	HTTP 1.1	Used if neither the return of status code 415 nor 400 would be justified, but processing of the request is rejected, for example, due to semantic errors (RFC 4918)
`423 Locked`	blocked	HTTP 1.1	The requested resource is currently locked (RFC 4918)
`424 Failed Dependency`	previous condition not met	HTTP 1.1	The request could not be carried out because it requires the success of a previous request.
`425 Unordered Collection`		HTTP 1.1	Defined in the WebDav Advanced Collections drafts, but not in the “Web Distributed Authoring and Versioning (WebDAV) Ordered Collections Protocol”
`426 Upgrade Required`	Update needed	HTTP 1.1	The client should switch to Transport Layer Security (TLS / 1.0).
`428 Precondition Required`	Precondition needed	HTTP 1.1	Not all prerequisites have been met for the request. This status code is intended to prevent problems caused by race conditions by manipulation or deletion only if the client requests this based on a current resource (for example, by supplying a current ETag header) ( RFC 6585 )
`429 Too Many Requests`	too many requests	HTTP 1.1	The client sent too many requests in a certain period of time. (RFC 6585)
`430 Request Header Fields Too Large`	Too many request headers	HTTP 1.1	It means that the server doesn’t want to process the request because of the excessive amount of large request and response headers. The header size can be decreased so that the server can respond to the requestor. 430 Status Code also can be seen if one of the response headers is wrong. The server will show which headers are too large or in the wrong format to the requestor.
`431 Request Header Fields Too Large`	Length too big	HTTP 1.1	The maximum length of a header field or the entire header was exceeded (RFC 6585)
`451 Unavailable For Legal Reasons`	Resource not available due to legal reasons	HTTP 1.1	The resource is not available (in a particular country) for legal reasons (e.g. copyright or censorship). Introduced in 2015 and after Ray Bradbury’s novel Fahrenheit 451 named.

HTTP 400 Status Codes

5xx: Server HTTP Status Codes

You can see the meaning, definition, and function of 500 Status Codes, in the table below.

500 Codes for HTTP Status Codes	Designation of HTTP Status Code	HTTP version	Description of HTTP Status Code
`500 Internal Server Error`	Internal server error	HTTP 1.1	The server detects an internal error and therefore cannot process the request. Errors in programs on the server, such as PHP scripts, can provoke this answer.
`501 Not Implemented`	Not Implemented	HTTP 1.1	The server does not have the functionality to process the request. This answer is appropriate if the HTTP method used in the request is not recognized or supported.
`502 Bad Gateway`	Bad portal	HTTP 1.1	The server, in this case, a proxy, cannot execute the request because an error has occurred in the further course.
`503 Service Unavailable`	service not available	HTTP 1.1	The service is currently unavailable. This can happen due to high data traffic or due to maintenance work.
`504 Gateway Time-out`	Portal time out	HTTP 1.1	The proxy cannot process the request due to a timeout.
`505 HTTP Version Not Supported`	HTTP version not supported	HTTP 1.1	The requested HTTP version is not supported.
`506 Variant Also Negotiates`	Circular reference	HTTP 1.1	The content agreement of the request results in a circular reference.
`507 Insufficient Storage`	Server memory is insufficient	HTTP 1.1	The request could not be processed because the server’s storage space is currently no longer sufficient.
`508 Loop Detected`	Infinite loop	HTTP 1.1	The operation was not carried out because the execution would have run in an endless loop. Defined in the binding extension for WebDAV in accordance with RFC 5842, because cyclic paths to WebDAV resources can arise through binding.
`509 Bandwidth Limit Exceeded`	Transfer rate/data volume exceeded	HTTP 1.1	The request was rejected because otherwise the available bandwidth would be exceeded (unofficial extension of some servers).
`510 Not Extended`	too little information	HTTP 1.1	The request does not contain all the information that the requested server extension expects (RFC 2774)
`511 Network Authentication Required`	Identification needed	HTTP 1.1	The client must first authenticate to gain access to the network (RFC 6585)

HTTP 500 Status Codes

Last Thoughts on HTTP Status Codes and SEO

HTTP Status Codes are important signals for a Search Engine. Every Status Code has a different meaning. Every different HTTP Status Codes are different hints, commands, and signals for the crawling, indexing, and ranking algorithms of the Search Engine. Thus, a Holistic SEO should think more analytically and carefully about HTTP Status Codes. A 308 and 301 Redirect can have the “same meaning” but, they can have different signals for the indexing or canonicalization. For instance, in 308 Redirect HTTP Status Code, a request has to have the “POST” method instead of “GET”. Thus, even if they have the same meaning, they can function in different ways.

As Holistic SEOs, we will continue to improve our information and guideline for HTTP Status Codes. With great information and experience related to the HTTP Status Code, a Holistic SEO can make better log analysis and save the crawl quota of the web entity. Also, analyzing the servers’ uptime or response headers for different user-agents can be understood via HTTP Status Codes.

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Koray Tuğberk GÜBÜR

Owner and Founder at Holistic SEO & Digital

Koray Tuğberk GÜBÜR is the CEO and Founder of Holistic SEO & Digital where he provides SEO Consultancy, Web Development, Data Science, Web Design, and Search Engine Optimization services with strategic leadership for the agency’s SEO Client Projects. Koray Tuğberk GÜBÜR performs SEO A/B Tests regularly to understand the Google, Microsoft Bing, and Yandex like search engines’ algorithms, and internal agenda. Koray uses Data Science to understand the custom click curves and baby search engine algorithms’ decision trees. Tuğberk used many websites for writing different SEO Case Studies. He published more than 10 SEO Case Studies with 20+ websites to explain the search engines. Koray Tuğberk started his SEO Career in 2015 in the casino industry and moved into the white-hat SEO industry. Koray worked with more than 700 companies for their SEO Projects since 2015. Koray used SEO to improve the user experience, and conversion rate along with brand awareness of the online businesses from different verticals such as retail, e-commerce, affiliate, and b2b, or b2c websites. He enjoys examining websites, algorithms, and search engines.

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What are the HTTP Status Codes: Everything You Need to Know