Open port
（重定向自Closed port）
In security parlance, the term open port is used to mean a TCP or UDP port number that is configured to accept packets. In contrast, a port which rejects connections or ignores all packets directed at it is called a closed port.
Ports are an integral part of the Internet's communication model — they are the channel through which applications on the client computer can reach the software on the server. Services, such as web pages or FTP, require their respective ports to be "open" on the server in order to be publicly reachable.