A.255.255.255.252
B.255.255.255.128
C.255.255.255.0
D.255.255.255.192
A.255.255.255.252
B.255.255.255.248
C.255.255.255.240
D.255.255.255.224
A.255.255.255.0
B.255.255.255.128
C.255.255.255.192
D.255.255.255.224
A.定义了子网中网络号的位数
B.子网掩码可以把一个网络进一步划分成几个规模相同的子网
C.子网掩码用于设定网络管理员的密码
D.用户不能自己设置子网掩码