子网与掩码

虽说这个也可以现场计算，但是混个面熟也未尝不可。

  Here is a table of network masks:


       Short   Full                    Maximum         Comment
         Form    Form                    #Machines

       /8      /255.0.0.0              16,777,215      Used to be called an `A-class'
       /16     /255.255.0.0            65,535          Used to be called an `B-class'
       /17     /255.255.128.0          32,767
       /18     /255.255.192.0          16,383
       /19     /255.255.224.0          8,191
       /20     /255.255.240.0          4,095
       /21     /255.255.248.0          2,047
       /22     /255.255.252.0          1,023
       /23     /255.255.254.0          511
       /24     /255.255.255.0          255             Used to be called a `C-class'
       /25     /255.255.255.128        127
       /26     /255.255.255.192        63
       /27     /255.255.255.224        31
       /28     /255.255.255.240        15
       /29     /255.255.255.248        7
       /30     /255.255.255.252        3