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Here are some simple steps to do it, using Network Address Translation (NAT)
1. Enable packet forwarding . Make it permanent by adding “net.ipv4.ip_forward = 1″ to /etc/sysctl.conf

echo "1" > /proc/sys/net/ipv4/ip_forward

2. Enable iptables to handle NAT. ( eth0 is the public connection )

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

3. Save iptables settings

service iptables save

4. Done. Check by routing table by verifying “netstat -nr” and iptables rules by “iptables -L”
This is a simple method for Redhat Enterprise Linux. See iptables docs for more detailed info.

Sumber: http://unixfoo.blogspot.com/2008/02/how-to-configure-linux-machine-as.html