區域是中樞區域,不能是存根區域.
在主機有多個IP位址的情況下表示屬於主機的所有IP位址，因此如果只想要一個IP位址接受連接，用該IP位址替換。
0 means all IP address that belong to the host in case the host has multiple IP addresses, so if only one IP address is desired to accept connections, one could replace with that IP address.
