子网划分

```
A类地址：10.0.0.0～10.255.255.255
B类地址：172.16.0.0 ～172.31.255.255  #172.16.0.0/12
C类地址：192.168.0.0～192.168.255.255
```
### 子网掩码-掩码位-反掩码 对照表
```
反掩码：             掩码位：     正掩码：
127.255.255.255      =    1    =      128.0.0.0
63.255.255.255       =    2    =      192.0.0.0
31.255.255.255       =    3    =      224.0.0.0
15.255.255.255       =    4    =      240.0.0.0
7.255.255.255        =    5    =      248.0.0.0
3.255.255.255        =    6    =      252.0.0.0
1.255.255.255        =    7    =      254.0.0.0
0.255.255.255        =    8    =      255.0.0.0
0.127.255.255        =    9    =      255.128.0.0
0.63.255.255         =    10 =      255.192.0.0
0.31.255.255         =    11 =      255.224.0.0
0.15.255.255         =    12 =      255.240.0.0
0.7.255.255          =    13 =      255.248.0.0
0.3.255.255          =    14 =      255.252.0.0
0.1.255.255          =    15 =      255.254.0.0
0.0.255.255          =    16 =      255.255.0.0
0.0.127.255          =    17 =      255.255.128.0
0.0.63.255           =    18 =      255.255.192.0
0.0.31.255           =    19 =      255.255.224.0
0.0.15.255           =    20 =      255.255.240.0
0.0.7.255            =    21 =      255.255.248.0
0.0.3.255            =    22 =      255.255.252.0
0.0.1.255            =    23 =      255.255.254.0
0.0.0.255            =    24 =      255.255.255.0
0.0.0.127            =    25 =      255.255.255.128
0.0.0.63             =    26 =      255.255.255.192
0.0.0.31             =    27 =      255.255.255.224
0.0.0.15             =    28 =      255.255.255.240
0.0.0.7              =    29 =      255.255.255.248
0.0.0.3              =    30 =      255.255.255.252
0.0.0.1              =    31 =      255.255.255.254
0.0.0.0              =    32 =      255.255.255.255
```