iptables NAT prerouting & postrouting

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http://www.unix.com – Good morning, I'm a newbie of iptables and as far as I've seen on tutorials on the Internet it seems that both prerouting and postrouting NAT chains are undergone both by a packet that goes from an internal LAN to the Internet and of a one that goes in the opposite direction (from the Internet to the LAN). AS NAT prerouting is DNAT, i.e. changing of the destination IP address and NAT postrouting is SNAT, i.e. changing of the source IP address, I naturally think that for a packet that exits the internal LAN and goes to the Internet only NAT postrouting is necessary while for a packet that from (HowTos)