iptables/forwarding connection
 

I have no clue as to what iptables command to run so I need some help. I am connecting to my network though a VPN client. I need to have my Cent OS4 box forward a connection. The reason for this is my mail server is here and then to login to my As400 I have to shutdown the VPN and bring it back up on a different site. I was wondering if I could just connect to my local work and have my computer inside route me accross the VPN. The trick is I have to connect on one port and forward to another.

Server address aab.bbb.ccd.12 Port 24 to be forwarded to aab.bbc.dcc.99 Port 23.

Can someone help me do this?