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Stateful Packet InspectionDuring the conversation between a host on the internal network and a host on the external network, a firewall tracks the state of the conversation between the hosts. It does this to know when to allow responses from the destination computer through the firewall. When a host computer tries to talk with another host computer on the other side of the firewall, a socket is created which includes the source ip address and the randomly selected source port. The firewall stores this socket in it's cache so that when a response is received from the destination host it knows that this traffic is acceptable and passes it through the firewall.
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