The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)                      9  TCP/IP Hypertext Transfer Protocol (HTTP)                           9  HTTP Messages, Message Formats, Methods and Status Codes

HTTP Methods 1 2 3 4 HTTP Message Headers

Status Code Format

HTTP status codes are three digits in length and follow a particular format where the first digit has particular significance. Unlike FTP and the others, the second digit does not stand for a functional grouping; the second and third digits together just make 100 different options for each of the categories indicated by the first digit. Thus, the general form of an HTTP status code is “xyy”, where the first digit, “x”, is specified as given in Table 274.

In each of these five groups, the code where “yy” is “00” is defined as a “generic” status code for that group, while other two-digit combinations are more specific responses. For example, “404” is the well-known specific error message that means the requested resource was not found by the server, while “400” is the less specific “bad request” error. This system was set up to allow the definition of new status codes that certain clients might not comprehend. If a client receives a strange code, it just treats it as the equivalent of the generic response in the appropriate category. So if a server response starts with the code “491” and the client has no idea what this is, it just treats it as a 400 “bad request” reply.

The reason phrase is a text string that provides a more meaningful description of the error for people who are bad at remembering what cryptic codes stand for (which would be most of us!) The HTTP standard includes “sample” reason phrases for each status code, but these can be customized by the administrators of a server if desired. When a server returns a more detailed HTML error message in the body of its response message, the reason phrase is often used for the “title” tag in that message body.

HTTP Methods 1 2 3 4 HTTP Message Headers