HTTP: The Definitive Guide

Status Codes

HTTP status codes are classified into five broad categories, as shown earlier in Table 3-2. This section summarizes the HTTP status codes for each of the five classes.

The status codes provide an easy way for clients to understand the results of their transactions. In this section, we also list example reason phrases, though there is no real guidance on the exact text for reason phrases. We include the recommended reason phrases from the HTTP/1.1 specification.

100-199: Informational Status Codes

HTTP/1.1 introduced the informational status codes to the protocol. They are relatively new and subject to a bit of controversy about their complexity and perceived value. Table 3-6 lists the defined informational status codes.

Table 3-6. Informational status codes and reason phrases

Status code	Reason phrase	Meaning
100	Continue	Indicates that an initial part of the request was received and the client should continue. After sending this, the server must respond after receiving the request. See the Expect header in Appendix C for more information.
101	Switching Protocols	Indicates that the server is changing protocols, as specified by the client, to one listed in the Upgrade header.

The 100 Continue status code, in particular, is a bit confusing. It’s intended to optimize the case where an HTTP client application has an entity body to send to a server but wants to check that the server will accept the entity before it sends it. We discuss it here in a bit more detail ...