Philips CPWBS154 - Manual (Page 49)

ENGLISH 49
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Stateful Packet Inspection
This is called a ‘stateful’ packet inspection because it examines the contents
of the packet to determine the state of the communications; i.e., it ensures
that the stated destination computer has previously requested the current
communication.This is a way of ensuring that all communications are
initiated by the recipient computer and are taking place only with sources
that are known and trusted from previous interactions. In addition to being
more rigorous in their inspection of packets, stateful inspection firewalls also
close off ports until connection to the specific port is requested.
When particular types of traffic are checked, only the particular type of
traffic initiated from the internal LAN will be allowed. For example, if the
user only checks ‘FTP Service’ in the Stateful Packet Inspection section, all
incoming traffic will be blocked except for FTP connections initiated from
the local LAN.