Request received, continuing process. The following class of status codes indicate that there is a response which consists of the Status-Line and optional headers that terminates via an empty line. Because HTTP/1.0 did not define any 1xx status codes, servers must not send a 1xx response to an HTTP/1.0 client except under testing conditions.

The following class of status codes indicates the action requested by the client was completely received, understood, accepted and processed successfully.

The following class of status code indicates the client is required to make additional action in order to complete the request. These status codes are usually utilized for URL redirection. User agents may carry out the additional action with no user interaction provided the method used in the second request is GET or HEAD. A UA should not automatically redirect a request more than five (5) times. It is deemed that such redirections may indicate an infinite loop.

These are your usual error status codes, better known for the 4xx number combination. Except when responding to a HEAD request, the server is required to include an entity containing details of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. UAs should display any included entity to the user.

These status codes informs you that the server failed to complete an apparently valid request. Response status codes starting with number “5” indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server must include an entity detailing the error situation, and indicate whether it is a temporary or permanent condition. Likewise, UAs are required to display any included entity to the user. These response codes are applicable to any request method.

Non-standard, these status codes are not specified by any RFC, but are used by third-party services to provide semantic or RESTful error responses

The IIS expands the 4xx error space to signal errors with the client’s request.

The Nginx web server software expands the 4xx error space to signal issues with the client’s request.

CloudFlare’s (CDN Service) reverse proxy service expands the 5xx error space to signal issues with the origin server.