In addition to its relative speed when compared with other types of scans, its major advantage is its ability to scan through stateless firewall or ACL filters. Such filters are configured to block access to ports usually by preventing SYN packets, thus stopping any attempt to 'build' a connection. XMAS packets, like out-of-state FIN or ACK packets, tend to pass through such devices undetected. Because open ports are inferred via no responses being generated, one cannot distinguish an open port from a filtered port without further analysis. For instance, XMAS scanning a system protected by a stateful firewall may indicate all ports being open. Because of their obvious rule-breaking nature, XMAS scans are flagged by almost all intrusion prevention or intrusion detection systems.