NAT, iptables and problematic ports

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http://serverfault.com – I am building a small office network with virtual machines. My schema is this: Computer A: gateway, ip, iptables used for NAT [eth0=public internet dhcp, dhcp; eth1=gateway] Computer B: client, ip, using gateway from Computer A. NAT is working, and Computer B can access the internet using the A's gateway. I redirected some incoming ports from A to B (for instance, if A receives a request to port 80, it goes automatically to Computer B's Apache). The thing is that I do not really understand how to open/close ports for Computer B from Computer A. I know how to close a port: (HowTos)