|Several Internet standards use three-digit reply codes. How are these three-digit reply codes made up?
The first code partitions the reply code space into main topics, in SMTP for example these are:
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:
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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