单项选择题
A.open报文B.update报文C.keepalive报文D.notification报文
A.10.254.255.23B.10.254.255.20C.10.254.255.1D.10.254.255.111
A.172.16.2.0B.172.16.2.128C.172.16.2.63D.172.16.2.252