ls command in ftp> not working!

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 command. ftp: bind: Address already in use ftp> The below is for a different remote machine where I have to send some files. *But as ls on ftp> isn't working, how will I transfer files from my local box to that box because I can't be confirmed without ls whether the files have been transferred or not.* ravbholua@ravbholua-Aspire-5315:~$ ftp 125.21.153.140 Connected to 125.21.153.140. 220---------- Welcome to Pure-FTPd [TLS] ---------- 220-You are user number 1 of 10 allowed. 220-Local time is now 04:34. Server port: 21. 220-This server supports FXP transfers 220 You will be disconnected after 2 minutes of inactivity. Name (125.21.153.140:ravbholua): peacenews 331 User peacenews OK. Password required Password: 230 OK. Current restricted directory is / Remote system type is UNIX. Using binary mode to transfer files. ftp> ls 200-FXP transfer: from 123.63.112.168 to 10.215.10.80 200 PORT command successful Please note that for the above machine, once I ran ls on ftp>, the prompt didn't come back. On both the remote machines, I got different output when executed ls on ftp>