TABLE 17-2   SECOND-DIGIT FTP REPLY CODES
Second Digit	Category Description
x0x	If the first digit indicates an error, this indicates it is a syntax error. If the first digit indicates success, this indicates the command syntax was understood.
x1x	This reply is for informational purposes.
x2x	This reply relates to the control or data connection status.
x3x	Replies with 3 as the second digit refer to authentication messages.
x4x	These replies are currently unused.
x5x	This reply relates to the server’s file system in some way.