单项选择题
A.数据链路层 B.网络层 C.传输层 D.应用层
A.0111 B.1111 C.101100111 D.101101111
A.172.30.128.0/21 B.172.30.128.0/22 C.172.30.130.0/22 D.172.30.132.0/23