IP Block Size and usable host

Prefix
Mask
No of  Usable Host
Block Size
/30
252
2
4
/29
248
6
8
/28
240
14
16
/27
224
30
32
/26
192
62
64
/25
128
126
128
/24
0
254
256
/23
254.0
510
512
/22
252.0
1022
1024
/21
248.0
2046
2048
/20
240.0
4094
4096
/19
224.0
8190
8192
/18
192.0
16382
16384
/17
128.0
32766
32768
/16
0.0
65534
65536

Host/Subnet size Table

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