Firewall type which implements filter rules on the basis of IP addresses and port numbers. If the connection is permissible under the rules concerned, the packet filter will allow the connection’s data to pass like a router. This means that a packet filter is able to guard against attacks on the IP level only if additional protection mechanisms take effect. What is called "stateful inspection" provides additional protection. A classical packet filter has no access to application data and is therefore unable to detect any attacks on the application level (e.g. viruses).