Can't connect to ISP's FTP server. What's wrong?

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https://bbs.archlinux.org – For the past six months I have been completely unable to connect to my ISP's FTP server. I can connect to any other FTP server; but not my ISP's.I've tried opening various ports on my router. I've tried putting the client computer in a DMZ, completely open to the internet. I've tried logging in from different locations with service provided by different ISPs. No matter what, the FTP client just hangs for a while and then times out. It matters not what configuration, what port, or what operating system.The ISP swears the problem is on my end, not theirs.Is there somethi (HowTos)