Using iptables, how can I get ftp connections to be forwarded/redirected from the 'public' IP address to an internal 'private' address ?

I tried:
iptable -t nat -A PREROUTING -p tcp -s publicadres --dport 21 -j DNAT --to

but that didn't seem to do anything, as the ftp to publicadres did not get redirected.

Is there a tool other than iptables that will do this ? (already tried simpleproxy, didn't work)