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Stateful Inspection

A firewall technology that ensures that all inbound packets are the result of an outbound request. For example, if you click on a link to a Web page, you are making an HTTP request to a specific URL. All packets coming from that URL would pass the stateful inspection and be accepted. Every so many minutes, your e-mail program queries the mail server, and returning packets from that server are allowed.

Also called "stateful packet inspection" (SPI), it was designed to help prevent an attacker from sending harmful, and certainly unrequested, packets into the network. Stateful inspection is the norm and generally a major component of every personal firewall including the firewalls built into Windows.

Stateful inspection causes problems with videoconferencing and VoIP, in which a user outside the network wants to initiate a communication with a user inside the network. Various techniques are used to work around this (see STUN, UpnP and port forwarding).