Configuring iptables --help

Hi,

i have linux gateway system with one public ip(eth1) and i am using
internet in LAN without any issue.
Now i want to configure 5 more public ip's to the eth1 and i want to
forward ftp request to different mechines in local LAN as below.

( glip -- global ip loip -- local ip )

ie. glip1 --> loip1
glip2 ---> loip2
glip3 --> loip3 .....
if i ftp to glip1 it should forward to loip1,
if i ftp to glip2 it should forward to loip2.

How to write the iptables?. Please help me in writing the iptables.

Thanks in advance.