NAT ports - how do they work?

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http://serverfault.com – I have the following network schema: Computer A: three nics: NIC 1 (eth0): dhcp, public internet NIC 2 (eth1): static, gateway for Computer B NIC 3 (eth2): static, gateway for Computer C Computer B: static, using gateway (NIC 2). Computer C: static, using gateway (NIC 3). So I applied this to get NAT working: iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE Every computer can connect to the internet now. I have been applying rules to the main computer (Computer A), like dropping connections to (HowTos)