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14.7. Mask



舉例說明該算法。
例:給定一 class c address : 192.168.5.0 ,要求劃分20個子網,每個子網5個主機。
解:因為4 <5 < 8 ,用256-8=248 ---->即是所求的子網掩碼,對應的子網數也就出來了。這是針對C類地址。
針對B類地址的做法。對於B類地址,假如主機數小於或等於254,與C類地址算法相同。對於主機數大於254的,如需主機 700台,50個子網(相當大了),512 < 700< 1024
256-(1024/256)=256-4=252 ---->即是所求的子網掩碼,對應的子網數也就出來了。上面256-4中的4(2的2次冪)是指主機數用2進製表示時超過8位的位數,即超過2位,掩碼為剩餘的前6位,即子網數為2(6)-2=62個。

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.