Pure-ftpd already open for IPv4 and IPv6.
Investigate to /etc/xinetd.d/pureftp. If you found 'service ftp' that mean this config still original from rpm. But, if you click update in 'admin > ftp configure', you will see 'service pureftp' instead 'service ftp'. The same why in /etc/services, 'ftp' will be replace to 'pureftp'.
This is a trick to make ftp using port other than 21.