I've just setup Redhat Linux
The network is running, via eth0 and ppp0 is the
internet connection. Pinging from any machine to
any other machine on network works.

Main server + internet connection =
other machines ->

I was told to modifying /etc/sysctl.conf,
net.ipv4.ip_forward = 1

I was given the following command to use to setup the
ip forwarding,
"iptables -t nat -I POSTROUTING -s -d
service iptables save

but it isn't working properly, I saw several other posts talking
about the similar types of things, but was unable to fathom
it out.

Do I also need to change the firewall, and allow/disallow things
in there. System Level Configuration shows
Security level = Enable Firewall
Trusted Services = WWW
Trusted Devices = eth0
Other Ports 443:tcp