Forward all ports incoming to a specific IP to internal IP address

Pablo is absolutely right, but I might as well provide a sample line here, which assumes your eth0:alias address is 1.2.3.5 and your internal IP of choice is 10.1.1.2:

iptables -t nat -A PREROUTING  -p tcp -i eth0 -d 1.2.3.5 -j DNAT --to-destination 10.11.1.2

If you want UDP as well, either have a second line for udp or just remove -p tcp.

This answer is no substitute for reading and understanding a good HOWTO, but it gives you some idea of where to start looking.

Hi I believe this step by step how to should be what you need.

http://www.howtoforge.com/nat_iptables

You want to go with NAT instead of PAT.