HTTP status codes: Definition, examples and meaning for SEO

HTTP status codes are number codes used by servers to indicate how a request was handled by a client. They are sent by servers in response to a request from a client and indicate whether the request was successful or whether an error occurred. There are hundreds of different HTTP status codes that have different meanings. Some of the most common status codes are 200 OK, 404 Not Found and 500 Internal Server Error.

HTTP status codes play an important role in SEO as they help search engine bots and other tools understand how the website is performing and whether it can be indexed easily. For example, the status code 200 OK is displayed when a request has been successfully processed and the requested page has been found. The 404 Not Found status code is displayed if the requested page was not found. If search engine bots encounter many 404 errors on a website, this can result in the website appearing lower in the search results or even being removed from the search results completely. It is therefore important to ensure that the website is functioning properly and that the HTTP status codes are displayed correctly to improve SEO.

There are 5 groups of HTTP status codes:

1. Informational (1xx): These codes indicate that the request has been received and is being processed. Examples are 100 (Continue) and 101 (Switching Protocols).

2. Success (2xx): These codes indicate that the request was successful. Examples are 200 (OK) and 201 (Created).

3. Redirection (3xx): These codes indicate that further steps are required to complete the request. Examples are 301 (Moved Permanently) and 302 (Found).

4. Client Error (4xx): These codes indicate that an error has occurred on the client side. Examples are 400 (Bad Request) and 401 (Unauthorized).

5. Server Error (5xx): These codes indicate that an error has occurred on the server side. Examples are 500 (Internal Server Error) and 503 (Service Unavailable).

There are also some special HTTP status codes such as 204, 205, 206, 308, 426, 444, 449, 451, 495, 496, 497, 499, 511 and 520-524 that can be used in certain circumstances.

Since the status codes of group 1 (1xx) are purely informative, we will not go into them in detail. Details on the status codes can be found, for example, on Wikipedia.

Success (2xx): These codes indicate that the request was successful. Examples are 200 (OK) and 201 (Created).

Status code 200 (OK)

The HTML status code 200 means that a request was successful and the desired resource was successfully delivered. This status code is an indicator that the page is accessible for search engines to crawl and index. There is no necessary action to take because this code indicates a successful request.

Status code 201 (Created)

The HTML status code 201 means that a request was successful and a new resource was successfully created. This code is often used when a user has created a resource via a form or a file has been uploaded. In terms of SEO, as long as the new resource has a unique URL and is accessible to search engines, it will be crawled and indexed by search engines and can help improve the visibility of the website. There is no necessary action to take as this code indicates a successful request and the new resource is accessible.

Status code 202 (Accepted)

The HTML status code 202 means that a request has been received successfully but has not yet been processed. This code is often used when a request is processed asynchronously, e.g. for longer processing or actions that are performed in the background. In terms of SEO, as long as the processed resource has a unique URL and is accessible to search engines, it will be crawled and indexed by search engines and can help improve the visibility of the website. There is no necessary action to take as this code indicates a successful query and the edited resource is accessible when it is available.

Status code 203 (Non-Authoritative Information)

The HTML status code 203 means that the request was successful, but the information returned is not from the original source. This code is often used when the information comes from a source other than the one originally requested, e.g. a cache. In terms of SEO, as long as the returned information has a unique URL and is accessible to search engines, it will be crawled and indexed by search engines and can help improve the visibility of the website. There is no necessary action to be taken as this code indicates a successful query and the returned information is accessible.

Status code 204 (No Content)

Status code 204 stands for "No Content". This status code is used to indicate that a request has been successfully processed but no content is returned. Here are three examples of the use of status code 204:

1. If a user requests a web page that has no content, the server could return a status code 204 to indicate that the request was successfully processed but no content is delivered.

2. When a user makes an API call to delete data on a server, the server might return a status code 204 to indicate that the request was successfully processed but no data is returned.

3. If a user sends a query to a search engine, such as Google, that does not return search results, Google's server might return a status code 204 to indicate that the query was successfully processed but no search results are returned.

In terms of SEO (Search Engine Optimisation), status code 204 means that the requested page has no content that could be indexed by search engines. If a page has no content, it will not be displayed in the search results and therefore has no influence on the ranking in the search engines. It is therefore important to ensure that any page that is to be indexed by the search engines actually has content.

Status code 205 (Reset Content)

The HTML status code 205 means that the request was successful and the client should reset the request form. This status code is often used when a user has submitted a form and it is necessary for the form to be reset to allow further input. In terms of SEO, as no content is returned, there is no direct impact on the visibility of the website. Developers should ensure that the form is reset properly and that there is a clear instruction for users on what to do next.

Status code 206 (Partial Content)

Status code 206 stands for "Partial Content". This status code is used when the server can only deliver part of the requested resource, such as a video that was stopped at a certain point in time and is now continuing to play. Here are three examples of the use of status code 206:

1. If a user is playing a video that is stored on a server and the video is interrupted because the user loses the connection, the server could return a status code 206 to indicate that only the remaining part of the video is being delivered.

2. If a user is downloading a large file and the connection is lost before the download is complete, the server could return a status code 206 to indicate that only the remaining part of the file is being delivered.

3. If a user makes an API call to retrieve data from a server but only needs part of the data, the server could return a status code 206 to indicate that only part of the data is supplied.

In terms of SEO (Search Engine Optimisation), the status code 206 has no direct effect, as it is usually only used when only part of a resource is delivered. However, if a page is not fully loaded, this could affect the search engine ranking, as the search engines cannot fully index the contents of the page. It is therefore important to ensure that all pages indexed by the search engines can be fully loaded.

Status code 207 (Multi-Status)

HTML status code 207 (multi-status) means that multiple status codes are returned in connection with a single request. This code is often used by WebDAV servers to return the results of multiple method calls or the results of multiple operations on a resource. In terms of SEO, since no content is returned, there is no direct impact on the visibility of the website. Developers should ensure that the multiple status codes are interpreted and processed properly and that the correct information is provided to users.

Status code 208 (Already Reported)

The HTML status code 208 (Already Reported) means that the requested resource has already been reported in a previous request and this message can be considered a duplicate. This code is often used by WebDAV servers to handle multiple requests to the same resource that have already been reported. In terms of SEO, there is no direct impact on the visibility of the website. Developers should ensure that the multiple requests are handled properly and that the correct information is provided to users.

Status code 226 (IM Used)

The HTML status code 226 (IM Used) means that the request was successful and the response is based on a previous request with the HTTP header "If-Modified-Since" or "If-None-Match". This code is often used to signal that a resource was previously retrieved and the current response is based on the conditions of the previous request. In terms of SEO, as long as the resource has a unique URL and is accessible to search engines, it will be crawled and indexed by search engines and can help improve the visibility of the website. There is no necessary action to take as this code indicates a successful query and the resource is accessible.

HTTP-3xx redirects are HTTP status codes that indicate that a request must be redirected to another URL in order to be processed successfully.

Status code 301 (Moved Permanently)

Status code 301 stands for "Moved Permanently". This status code is used to indicate that a resource has been permanently moved to a new location and all future requests should be redirected to the new location. Here are three examples of the use of status code 301:

1. If a website has moved and all requests to the old URL are to be redirected to the new location, the server could return a 301 status code to indicate that the resource has been permanently moved to the new location.

2. If a user sends an API request to an old endpoint that is no longer available, the server could return a 301 status code to indicate that the resource has been moved to a new endpoint and all future requests should be redirected to the new endpoint.

3. When a user performs a search query on a search engine, such as Google, and a search result is displayed that links to an old URL that is no longer available, Google's server could return a 301 status code to indicate that the resource has been moved to a new location and all future queries should be redirected to the new location.

In terms of SEO (Search Engine Optimisation), the status code 301 means that the resource has been permanently moved to a new location. When a resource is permanently moved, it is usually also removed from the search results and replaced by the new location. It is therefore important to ensure that when a resource is permanently moved, all links pointing to the old URL are redirected to the new URL so as not to affect the search engine ranking.

Status code 302 (Found - Moved Temporarily)

Status code 302 is an HTTP status code that indicates that a resource has been temporarily redirected to another URL. This means that if the client (e.g. a web browser) makes a request for the original URL, it will find the resource at a different URL.

Here are three possible examples of the use of status code 302:

1. When a website changes its URL, it could use status code 302 to redirect users from the old URL to the new one.

2. A website could use status code 302 to redirect users from a non-secure URL (http) to a secure URL (https).

3. A website could use status code 302 to redirect users from a mobile version of the website to the desktop version or vice versa, depending on which version is more suitable for the user.

In terms of SEO, the use of status code 302 can help search engines index the content of a resource at the new URL rather than at the original URL. However, it is important to note that status code 302 is temporary and search engines may have difficulty indexing the resource permanently at the new URL. In such cases, it may be better to use status code 301 (permanent redirect).

Status code 303 (See Other)

Status code 303 is an HTTP status code and means "See other". This code is used to indicate that the client should be redirected to another resource to process the request. The resource to which the client is redirected is indicated by the Location header.

An example of the use of status code 303 would be when a website has a link to a page that is only visible to logged-in users. If an unlogged-in user clicks on the link, the server could return the status code 303 and redirect the user to a login or registration page.

Another example would be if a website has a link to a page that is currently unavailable due to maintenance. The server could return the status code 303 and redirect the user to a page indicating that the resource is temporarily unavailable.

A third example would be if a website has a search engine that points to an external search engine. The server could return the status code 303 and redirect the user to the external search engine to perform the search.

For SEO, the status code 303 means that the originally requested page is not directly available, but that the user is redirected to another page. This could have an impact on the search engine position of the originally requested page, as search engine crawlers may not be able to crawl or index the original page. It is important to ensure that the redirected page is relevant, of high quality and represents the original page well to avoid any negative impact on SEO.

Status code 304 (Not Modified)

The HTML status code 304 (Not Modified) means that the request was successful, but the desired resource has not been modified since the client last retrieved it. This code signals to the client that it can retrieve the resource from the local cache instead of getting a new copy from the server. In terms of SEO, there is no direct impact on the visibility of the website as the resource has not been changed and therefore no new content is available to the search engines. However, developers should ensure that the HTTP header "Cache-Control" and "Last-Modified" are used properly to signal to the browser and search engines when a resource was last modified and how long it is allowed to remain in the cache.

Status code 305 (Use Proxy)

The HTML status code 305 (Use Proxy) means that the request was successful, but the client should resend the request via a specified proxy. This code is often used when the server has limited access to a resource and access must be provided via a proxy. In terms of SEO, there is no direct impact on the visibility of the website. However, developers should ensure that the client is properly prompted to resend the request via the specified proxy and that access to the resource is not restricted to ensure that search engines can crawl and index the resource.

Status code 306 (Unused)

The HTML status code 306 (Unused) used to be used to indicate that the request was successful, but the client should resend the request via another proxy. However, this code is no longer in use and has been removed in HTTP/1.1. It is no longer a valid HTTP status code. Developers should instead use another status code such as 307 or 308 to perform the desired action.

Status code 307 (Temporary Redirect)

Status code 307 is an HTTP status code that states that the request will be forwarded to the server-side location where it was originally made. This means that the client remains connected to the original location while the server forwards the request to another location. Some possible reasons why a server might return a 307 status code are:

1. The content targeted by the request has been temporarily moved to another location.
2. The content is only available under certain conditions and the server must forward the request to a place where the conditions can be checked.
3. The server wants to prevent the client from retrieving the content directly, but wants the client to retrieve a copy of the content from another location.

As far as SEO is concerned, a 307 status code can help a website improve its search engine ranking by preventing content from being duplicated by other websites. When a server returns a 307 status code, the client is prompted to retrieve the content from the location where it was originally placed, rather than from another website. This helps to ensure that search engines index the content of the original website and not the content of another website that may duplicate the same content.

Status code 308 (Permanent Redirect)

The status code 308 Permanent Redirect is an HTTP status code that indicates that a request is permanently redirected to another URL. Here are three examples:

1. A website has been moved from www.example.com to www.example.net. When someone visits www.example.com, they are automatically redirected to the new URL www.example.net.

2. A website has been moved from example.com to www.example.com. In this case, too, the user is redirected to www.example.com when calling up example.com.

3. A website offers different language versions. If someone calls up the German version (www.example.com/de), they are redirected to the English version (www.example.com/en) if this is the default.

For SEO, it is important that the status code 308 is used carefully, as search engines react to redirects when indexing web pages. If a website is permanently redirected, it could be difficult for search engines to find the original URL and include it in the search index. It is therefore important that website operators ensure that all redirects are implemented correctly and that the original URL is not lost.

The main difference between 301 and 308 is that the 301 completes the process of permanent redirection, while the 308 continues this process. This means that when the 301 status code is used, the original URL is no longer used and all links must be changed to the new URL. The 308, on the other hand, means that the original URL can still be used, but it is recommended to change links to the new URL.

In terms of SEO, there is not much difference between 301 and 308, as both indicate that a website has moved and search engines should therefore include the new URL in the search index. However, it is recommended to use the 301 status code as it completes the process of permanent redirection and it is less likely that errors will occur.

HTTP client errors are errors triggered by the client when a request is sent to a server. These error codes usually start with "4xx" and can indicate problems such as incorrect requests, missing requests or problems with authentication. Some examples of HTTP client error codes are 400 Bad Request, 401 Unauthorised, 403 Forbidden, 404 Not Found and 409 Conflict. These error codes inform the server that the error occurred at the client and not at the server and that the server is unable to successfully process the request.

Status code 400 (Bad Request)

Status code 400 means that the server could not process the client's request because the request was invalid or incomplete. Here are three examples of the use of status code 400:

1. The request contains invalid syntax: If a client sends a request to the server that does not comply with the valid syntax conventions, the server may return the status code 400.

2. The request contains invalid data: If a client sends a request to the server that contains invalid or inconsistent data, the server may return status code 400.

3. The request is incomplete: When a client sends a request to the server that contains required data or information, the server may return status code 400.

In terms of SEO, the status code 400 means that search engines have difficulty indexing the requested pages. This can result in the affected pages ranking lower in search results. It is important that websites ensure that their requests are valid and complete to ensure that search engines can index them easily.

Status code 401 (Unauthorised)

Status code 401 means that the server cannot process the client's request because the client has not provided valid credentials. Here are three examples of the use of status code 401:

1. The client has not provided valid credentials: If a client sends a request to a protected resource but does not provide valid credentials, the server may return status code 401.

2. The client does not have permission for the requested resources: If a client sends a request to a resource for which it does not have permission, the server may return the status code 401.

3. The client does not have valid cookies: If a client sends a request to a protected resource that relies on the client having valid cookies but does not, the server may return status code 401.

In terms of SEO, the 401 status code means that search engines have difficulty indexing the requested pages. This can result in the affected pages ranking lower in search results. It is important that websites ensure that they provide valid credentials and permissions to ensure that search engines can index them easily.

Status code 402 (payment required)

The HTML status code 402 (Payment Required) means that the request was successful but payment is required to gain access to the requested resource. This code is often used when access to a protected resource can only be granted after successful payment. In terms of SEO, there is no direct impact on the visibility of the website as the resource is not accessible unless the payment has been successfully made. Developers should ensure that payment processing works properly and that the resource is accessible after successful payment to ensure that search engines can crawl and index the resource.

Status code 403 (Forbidden)

Status code 403 means that the server has successfully received the client's request but will not process the request for reasons usually related to security. Here are three examples of the use of status code 403:

1. The client does not have permission for the requested resources: If a client sends a request to a resource for which it does not have permission, the server may return the status code 403.

2. The request violates the server's security policy: If a client sends a request to the server that violates the server's security policy, the server may return the status code 403.

3. Access to the resource is prohibited for other reasons: In some cases, the server may return the status code 403 if access to a resource is prohibited for other reasons, for example due to licence conditions or for legal reasons.

Access to the resource is prohibited for other reasons: In some cases, the server may return the status code 403 if access to a resource is prohibited for other reasons, for example due to licence conditions or for legal reasons.

Status code 404 (Not Found)

The status code 404 means that the server has successfully received the client's request, but the requested resource was not found. Here are three examples of the use of status code 404:

1. The requested resource has been deleted or moved: If a resource requested by a client has been deleted or moved, the server may return the status code 404.

2. The URL is misspelled: If a client sends a request to a URL that is misspelled or points to a resource that does not exist, the server may return the status code 404.

3. The server could not find the resource: In some cases, the server may return the status code 404 if it cannot find the requested resource due to technical problems.

In terms of SEO, the 404 status code means that search engines are having difficulty indexing the requested pages. This can result in the affected pages not ranking in search results. It is important that websites ensure that their URLs are correct and that they provide all resources to be indexed by search engines to ensure that they can be indexed without problems.

Status code 405 (Method Not Allowed)

Status code 405 means that the server has successfully received the client's request, but the requested resource does not support the HTTP method used by the request. Here are three examples of the use of status code 405:

1. The resource does not support POST requests: If a client sends a POST request to a resource that does not support POST requests, the server may return the status code 405.

2. The resource does not support PUT requests: If a client sends a PUT request to a resource that does not support PUT requests, the server may return the status code 405.

3. The resource does not support DELETE requests: If a client sends a DELETE request to a resource that does not support DELETE requests, the server may return the status code 405.

In terms of SEO, the status code 405 means that search engines have difficulty indexing the requested pages. This can result in the affected pages ranking lower in search results. It is important that websites ensure that they provide all supported HTTP methods to ensure that search engines can index them easily.

What are POST requests?

POST requests are a type of HTTP request used to send data to a server for storage or processing. POST requests are often used to send form input to a server for storage or processing. Unlike GET requests, which usually perform read-only operations, POST requests can cause changes to resources or data on the server. POST requests are also suitable for sending large amounts of data to a server, as they have no restrictions on the size of the request

What are PUT requests?

PUT requests are a type of HTTP request used to send data to a server to replace or update an existing resource. PUT requests are often used to send form input to a server to update existing data. Unlike POST requests, which typically create new resources, PUT requests are used to update or replace existing resources. It is important to note that PUT requests require a previous resource to be updated or replaced. If no previous resource exists, it may be more appropriate to use a POST request to create a new resource.

What are DELETE requests?

DELETE requests are a type of HTTP request used to delete an existing resource from the server. DELETE requests are often used to send form input to a server to delete existing data. Unlike PUT requests, which update or replace existing resources, DELETE requests are used to remove resources from the server. It is important to note that DELETE requests require a previous resource to be deleted. If there is no previous resource, it may be more appropriate to use another HTTP method such as GET or POST.

Status code 406 (Not Acceptable)

Status code 406 is an HTTP status code indicating that the server cannot process the user's request because the request headers sent by the client are invalid. This means that the client may have sent a header that cannot be processed by the server or that the client sent a header that is not relevant to the request.

Here are three examples where status code 406 could occur:

1. A client sends a request to a server with the header "Accept-Language: German", but the server only supports English as a language. In this case, the server would return the status code 406 to indicate that the client's request cannot be processed.

2. A client sends a request to a server with the header "If-None-Match: abc123", but the server has no resource that matches this value. In this case, the server would return the status code 406 to indicate that the client's request cannot be processed.

3. A client sends a request to a server with the header "Accept: application/pdf", but the server has no resource available as a PDF. In this case, the server would return the status code 406 to indicate that the client's request cannot be processed.

In terms of SEO, the status code 406 means that search engines may have difficulty crawling and indexing certain pages on your website. If search engines receive the status code 406 for a page, it means that they may not be able to crawl and index the page, which may result in it not appearing in search results. It is therefore important that you ensure that all relevant pages on your website can be crawled and indexed by search engines to ensure that your content is visible to users.

Status code 407 (Proxy Authentication Required)

The HTML status code 407 (Proxy Authentication Required) means that the request was successful, but the client must authenticate with the proxy to gain access to the requested resource. This code is often used when access to a protected resource can only occur after successful authentication to the proxy. In terms of SEO, there is no direct impact on the visibility of the website as the resource is not accessible unless authentication to the proxy has been successfully performed. Developers should ensure that authentication to the proxy works properly and that the resource is accessible after successful authentication to ensure that search engines can crawl and index the resource.

Status code 408 (Request Timeout)

The HTML status code 408 (Request Timeout) means that the request could not be processed by the server within the expected time frame. This can happen if the request is too complex, the connection is slow or the server is overloaded. In terms of SEO, a high number of timeout errors can cause search engines to rank the website as unreliable and affect visibility. Developers should ensure that requests are processed properly and that server capacity is sufficient to ensure a fast response time.

Status code 409 (Conflict)

The HTML status code 409 (Conflict) means that the request could not be fulfilled because there was a conflict with the current state of the server. This can happen when a user tries to create a resource that already exists or when a user tries to change a resource that has already been changed by another user. In terms of SEO, there is no direct impact on the visibility of the website. Developers should ensure that conflicts are handled properly and that users are informed about the conflict and the available solutions.

Status code 410 (Gone)

Status code 410 means that a requested resource is no longer available and will not be available again. This status code is returned by a server when the requested resource has been permanently removed.

Here are three examples of when a status code 410 might be returned:

1. If a web page has been permanently removed and is not to be made available again, the server could return the status code 410 to signal that the page is no longer available.

2. If a blog post was permanently removed, the server could return the status code 410 to indicate that the post is no longer available.

3. If a user follows a link to a resource that has been permanently removed, the server could return the status code 410 to signal that the resource is no longer available.

In terms of SEO, the status code 410 means that search engine bots should no longer index the requested resource and display it in search engine results. If a page or post has been permanently removed and a status code 410 is returned, the page or post will no longer be displayed in search engine results.

The status code 404 means that the requested resource was not found. This status code is returned by a server when the requested resource was not found on the server.

In contrast, status code 410 means that the requested resource has been permanently removed and is no longer available. This status code is returned by a server when the requested resource has been permanently removed and will not be available again.

The main difference between these two status codes is that the 404 status code indicates that the requested resource cannot be found at the moment, while the 410 status code indicates that the requested resource has been permanently removed and is no longer available. In terms of SEO, the status code 404 means that search engine bots cannot find the requested resource, but there is a possibility that it will become available again in the future. Status code 410, on the other hand, means that the resource has been permanently removed and should no longer be displayed in search engine results.

Status code 429 (Too Many Requests)

Status code 429 means that the server is receiving too many requests from a particular user or from a particular IP address and is therefore refusing to process further requests from that user or from that IP address. This type of status code is often used to protect against denial-of-service attacks or to ensure that a server is not overloaded by a single user or IP address.

Here are three examples of when a status code 429 might occur:

1. If a user calls up a website that makes plenty of requests to the server, for example because it contains very large images or videos. The server could block the user with a status code 429 to protect itself from overload.

2. When a bot or a script tries to call up a web page quickly in succession in order to crawl it or collect data from it. The server might block the bot with a status code 429 to ensure that the website's traffic is not overloaded by a single user.

3. If a user accesses a web page that has already been blocked by another IP address or user, the server could block the user with a status code 429 to ensure that the user does not bypass what the server considers to be unwanted traffic.

For SEO, a status code 429 means that search engine bots or crawlers may have difficulty crawling or indexing the affected website. This could result in the website ranking lower in search results or not being displayed at all. It is therefore important to ensure that the server has enough resources to handle all requests from search engine bots and users to ensure that the website is found well by search engines.

Status code 499 (Client Closed Request)

The status code 499 means that the client has terminated the request prematurely. This status code is normally issued by the server when the client has cancelled the request before completion.

Some examples of the use of status code 499 are:

1. A user who has a long load time and closes the page before it has fully loaded.

2. A user who starts a search query and then closes the search bar before the result has been returned.

3. A user who has a long form submission and closes the form before completion.

For SEO, the status code 499 has no meaning, as it refers to the client and not the server. Google and other search engines are unlikely to consider this status code as a ranking factor, as it refers to user actions on the website and not to the quality of the website itself.

Status code 500 (Internal Server Error)

The status code 500 means "Internal Server Error". This means that the server that was supposed to process the user's request had an error that makes it impossible for it to process the request.

Here are three examples that could lead to a status code 500:

1. An error in the code of a web application that is executed on the server, for example a syntax error in a Python script.

2. A problem with the database, for example, when a query fails because a required table was not found.

3. A hardware fault on the server, for example, when the server has been shut down due to a power failure.

For SEO (search engine optimisation), a status code 500 means that the page crawled by the search engine is not reachable. If a page is not reachable, it is not indexed by the search engines, which means that it is not displayed in the search results. This can lead to the page's visibility in the search results decreasing, which in turn can lead to it receiving less traffic.

Status code 501 (Not Implemented)

The HTML status code 501 (Not Implemented) means that the request could not be processed by the server because the method requested in the request header is not supported. This can happen when the server receives a request with a method that it does not understand or that it has not implemented. In terms of SEO, there is no direct impact on the visibility of the website. Developers should ensure that the server processes the requests properly and that users are informed about the support of the available methods.

Status code 502 (Bad Gateway)

Status code 502 (also called "Bad Gateway") means that the server that received the request is acting as a gateway or proxy for the request and has received an invalid response from the server that was actually supposed to handle the request.

Here are three examples of when a 502 status code might occur:

1. When a server serving as a gateway for requests from users receives an invalid response from another server it is requesting to retrieve information for users.

2. When a server serving as a proxy for requests from users receives an invalid response from another server it is requesting to redirect information for users.

3. When a server serving as a gateway or proxy for user requests is unable to successfully process user requests due to maintenance or other problems.

What status code 502 means for SEO depends on how often it occurs and how long it lasts. If users frequently encounter status code 502 errors when trying to access the website, this could lead to them leaving the website and visiting another website instead. This could lead to a decrease in the visibility of the website in search engines, and therefore a decrease in traffic coming to the website via search engines. However, if the 502 status code only occurs occasionally and is quickly remedied, this should not have a lasting impact on SEO.

Status code 503 (Service Unavailable)

The status code 503 is an HTTP status code and means that the server you are calling is currently unable to process the request. This can happen, for example, if the server is overloaded or if maintenance work is being carried out.

Here are three examples where a status code 503 could be displayed:

1. You are trying to access a website that is being maintained by its hosting provider. In this case, you will see an error message with the status code 503 when you try to access the website.

2. You are performing search engine optimisation (SEO) and are trying to optimise a specific link on your website. If the server hosting the website is currently unable to process the request, the status code 503 is displayed.

3. You are trying to access a specific page on a website that is currently overloaded by a large surge of traffic. Also in this case, you may see an error message with the status code 503.

In terms of the SEO aspect, it can be harmful if a website frequently displays status codes such as 503, as this can be an indication to search engines that the website is not stable and may not be able to provide content regularly. This can lead to the website being found less easily in search engine results, which in turn can lead to it receiving less traffic.

Status code 522 (Connection Timed Out)

Status code 522 means that the server was unable to connect to the client server to process the client's request. An example of this status code would be when a client sends a request to a server but the server is unreachable due to network problems. Another example would be when a client sends a request to a server that is offline due to maintenance. A third example would be when a client sends a request to a server that is locked due to security concerns.

In terms of SEO, the status code 522 means that search engine bots have problems reaching and indexing the website. If the website cannot be reached by search engine bots, this can result in the website appearing lower in the search results or even being completely removed from the search results. It is therefore important to ensure that the website can be easily reached by search engine bots to improve SEO.

Status code 999 (Invalid Status Code)

The status code 999 is not a valid HTTP status code and is not defined by the HTTP protocols. There are no defined meanings or use cases for the status code 999. The status code 999 is output, for example, for external links from one's own page to LinkedIn.

Since the 999 status code is not part of the HTTP protocols, it has no impact on SEO. Search engine bots and other tools used to index websites only use the HTTP status codes defined in the HTTP protocols to determine the availability and state of websites.