该函数利用地址类别确定在地址字段中哪些八位组包含的是网络部分，哪些八位组是主机部分，从而提取出IP地址中的网络部分。-This function is used to determine the address type in the address field octet which contains part of the network which octet is the host part of IP address in order to extract the network portion.