各种路由协议Preference和掩码对照表

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