单项选择题
A.物理层 B.网络层 C.传输层 D.应用层
A.IEEE802.1 B.IEEE802.2 C.IEEE802.3 D.IEEE802.4
A.16 B.20 C.40 D.64
A.255.255.255.0 B.255.255.255.128 C.255.255.255.192 D.255.255.255.224
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