Example simple iptable ruleset

#! /bin/sh
# /etc/iptables.bak

# Let's save typing & confusion with variables
IPTABLES=/sbin/iptables

# Flush active rules and custom tables
$IPTABLES --flush
$IPTABLES --delete-chain

# set the defaults so that by-default incoming and outgoing packets are explicitly allowed;
$IPTABLES -P INPUT ACCEPT
$IPTABLES -P FORWARD DROP
$IPTABLES -P OUTPUT ACCEPT

Invoke and make these rules effective:

hostname ~ # sh /etc/iptables.bak

Resulting active rules:

hostname ~ # iptables -L

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy DROP)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

REMEMBER! If you like the ruleset, and want it to be in-effect the next time you start iptables (ie after a reboot), then you must:

hostname ~ # rc-update add iptables default
* iptables added to runlevel default

hostname ~ # /etc/init.d/iptables save
* Saving iptables state ...