Preface: A HTTP status code is issued from the server in response to a client's request made to the server. The five status code response classes are informational, successful, redirection, client error and server error.
Informational status codes indicate that the request initiated by the browser was received and is continuing.
Definition: The request has been received and has not yet been rejected by the server. The server intends on sending a final response once the request has been completed meaning that the server has received the request headers and client can now proceed to send the request body. To have a server check the request's headers, a client must send
Expect: 100-continue as a header in its initial request and receive a
100 Continue status code in response before sending the body.
100 Continue is defined in RFC 7231.
Definition: The request has asked the server to switch protocols. The server response indicates that it is switching protocol as requested. In this response, the server includes an
Upgrade response header to indicate the protocol it switched to.
101 Switching Protocols is defined in RFC 7231.
Definition: The request has been received and is being processed by the server, but no response is available yet. In the meantime, this response prevents the client from timing out and assuming the request was lost.
102 Processing is defined in RFC 2518.
Definition: The server is likely to send a final response with the header fields included in the informational response. Mainly intended to be used with the
Link header to allow the user agent to start preloading resources while the server is still preparing a response.
103 Early Hints is defined in RFC 8297.
Unofficial and customized non-standard responses defined by server softwares are not included in the list above.