Fedora NAT-ing

I have a internet connection with static IP address.
I have created a LAN, all network machines can connect to internet.

ICS machine ip configuration:
ip:172.16.103.XXX
DNS server:172.16.103.XX

Network machine ip configuration:-
ip:192.168.1.2
subnet mask:255.255.255.0
gateway:192.168.1.1
DNS server:172.16.103.XX

now every computer in my local network is broadcasting my static ip address that is 172.16.103.XXX
I obviously do not want that. I want each computer on my local network go to the internet with their own ip (like 202.78.233.154). Please help.

I have set a nating rule with fwbuilder.
it is -
$IPTABLES -t nat -A POSTROUTING -o em1 -s 192.168.1.0/24 -j SNAT --to-source 172.16.103.XXX

but no effect.
Please help.