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14.7. Mask



举例说明该算法。
例:给定一 class c address : 192.168.5.0 ,要求划分20个子网,每个子网5个主机。
解:因为4 <5 < 8 ,用256-8=248 ---->即是所求的子网掩码,对应的子网数也就出来了。这是针对C类地址。
针对B类地址的做法。对于B类地址,假如主机数小于或等于254,与C类地址算法相同。对于主机数大于254的,如需主机 700台,50个子网(相当大了),512 < 700< 1024
256-(1024/256)=256-4=252 ---->即是所求的子网掩码,对应的子网数也就出来了。上面256-4中的4(2的2次幂)是指主机数用2进制表示时超过8位的位数,即超过2位,掩码为剩余的前6位,即子网数为2(6)-2=62个。

Append :Host/Subnet Quantities Table

----------------------------------------------------------------------
Class A                   Effective  Effective
# bits        Mask         Subnets     Hosts
-------  ---------------  ---------  ---------
  2      255.192.0.0            2      4194302
  3      255.224.0.0            6      2097150
  4      255.240.0.0           14      1048574
  5      255.248.0.0           30       524286
  6      255.252.0.0           62       262142
  7      255.254.0.0          126       131070
  8      255.255.0.0          254        65536
  9      255.255.128.0        510        32766
  10     255.255.192.0       1022        16382
  11     255.255.224.0       2046         8190
  12     255.255.240.0       4094         4094
  13     255.255.248.0       8190         2046
  14     255.255.252.0      16382         1022
  15     255.255.254.0      32766          510
  16     255.255.255.0      65536          254
  17     255.255.255.128   131070          126
  18     255.255.255.192   262142           62
  19     255.255.255.224   524286           30
  20     255.255.255.240  1048574           14
  21     255.255.255.248  2097150            6
  22     255.255.255.252  4194302            2

Class B                   Effective  Effective
# bits        Mask         Subnets     Hosts
-------  ---------------  ---------  ---------
  2      255.255.192.0           2     16382
  3      255.255.224.0           6      8190
  4      255.255.240.0          14      4094
  5      255.255.248.0          30      2046
  6      255.255.252.0          62      1022
  7      255.255.254.0         126       510
  8      255.255.255.0         254       254
  9      255.255.255.128       510       126
  10     255.255.255.192      1022        62
  11     255.255.255.224      2046        30
  12     255.255.255.240      4094        14
  13     255.255.255.248      8190         6
  14     255.255.255.252     16382         2

Class C                   Effective  Effective
# bits        Mask         Subnets     Hosts
-------  ---------------  ---------  ---------
  2      255.255.255.192      2         62
  3      255.255.255.224      6         30
  4      255.255.255.240     14         14
  5      255.255.255.248     30          6
  6      255.255.255.252     62          2

*Subnet all zeroes and all ones excluded.
*Host all zeroes and all ones excluded.