First number

1xx 	Positive Preliminary reply

2xx 	Positive Completion reply

3xx 	Positive Intermediate reply

4xx 	Transient Negative Completion reply

5xx 	Permanent Negative Completion reply


Second number

x0x 	Syntax

x1x 	Information

x2x 	Connections

x3x 	Authentication and accounting

x4x 	Unspecified as of RFC 959

x5x 	File system




Responses:
I number)
* 2x positive completion reply
* 4x transient negative completion reply (command not executed)
* 5x permanent negative completion reply (errors)

II number)
* x0 syntax
* x1 information
* x2 connection
* x5 file system



20 OK					MEOK
21 not used				-
22 not used				-
25 file purged				MEFP

40 not used				-
41 not used				-
42 shutting down			MESD
45 requested file action not taken	MENT

50 syntax error				MESY
51 not used				-
52 not used				-
55 server error				MESE