Packet filters are IT-systems provided with a special software to filter the incoming and outgoing data traffic of a network. It is their task to forward or discard databases by means of the information stored in the header of the UDP/IP- respectively TCP/IP-Level. In advance, the user determines the filtering rules. Packet filters offer the opportunity of a NAT, a network address translation, where the individual IP-addresses are exchanged by that of the packet filter. This conceals the network structure of the protected network.