As described earlier, a single pass of either the host or port scanning information is only useful for the instant in time when the scan was executed.
如前所述，如果只进行一次扫描，那么主机或者端口扫描信息仅在执行该次扫描的时候是有用的。
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A convenient way to find a free port is to pass in a null address, which automatically binds the socket to the local host address and USES a free ephemeral port.