A filter that provides the basic protection mechanism for a firewall by determining what traffic passes through the firewall based on IP session details. This protects the secure network from outsiders who use unsophisticated techniques (such as scanning for secure servers) or even the most sophisticated techniques (such as IP address spoofing). IP filters are the base on which the other tools are constructed, providing the infrastructure in which they operate and denying access to all but the most determined cracker.