971031-3 Several Internet standards use three-digit reply codes. How are these three-digit reply codes made up?

Correct response:

The first code partitions the reply code space into main topics, in SMTP for example these are:

1yz Positive Preliminary reply
2yz Positive Completion reply
3yz Positive Intermediate reply
4yz Transient Negative Completion reply
5yz Permanent Negative Completion reply

The second code partitions the reply code space within the main topics. This is done in different ways in different protocols, as an example, SMTP uses:

x0z Syntax error
x1z Replies to requests for information
x2z Connection-related replies,
x3z Unspecified as yet.
x4z Unspecified as yet.
x5z Status of the receiving mail system

The third code is used to distinguish between different response codes having the same first and second digit.

Other protocols using similar kinds of response codes are for example FTP and HTTP.

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