An In-Depth Guide to iptables, the Linux Firewall
This is a great guide to iptables: https://www.booleanworld.com/depth-guide-iptables-linux-firewall/
iptables
IP Masquerading using iptables
http://billauer.co.il/ipmasq-html.html
The Beginner’s Guide to iptables, the Linux Firewall
https://www.howtogeek.com/177621/the-beginners-guide-to-iptables-the-linux-firewall/