ls command in ftp> not working!

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http://unix.stackexchange.com – I was practicing ftp but faced an issue: ls command isn't working on ftp> . Why? I checked on 2 remote servers but ls didn't work on either and gave different output when ls was executed. Please see below for the 2 remote boxes. The below shows my remote server where I installed vsftpd today. ravbholua@ravbholua-Aspire-5315:~$ ftp rs Connected to ravi.com. 220 (vsFTPd 3.0.2) Name (rs:ravbholua): 331 Please specify the password. Password: 230 Login successful. Remote system type is UNIX. Using binary mode to transfer files. ftp> pwd 257 "/home/ravbholua" ftp> ls 500 Illegal PORT comma (HowTos)