![]() Filtered: Some ports may be filtered by a firewall or intrusion prevention system (IPS).A TCP SYN packet to a closed port should result in a RST (reset) packet. Closed: If a port is closed, then attempts to communicate with it are considered an error by the computer.For example, an open port receiving a TCP SYN packet should respond with a SYN/ACK. Open: An open port where an application is listening for traffic should respond to a legitimate request.If a port is open, closed, or filtered by a network security solution, it will respond in different ways to a port scan, including: A port scanner, such as nmap, works by sending traffic to a particular port and examining the results.
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