IP Subnet Masks

Net bits Subnet mask total-addresses
/20 255.255.240.0 4096
/21 255.255.248.0 2048
/22 255.255.252.0 1024
/23 255.255.254.0 512
/24 255.255.255.0 256
/25 255.255.255.128 128
/26 255.255.255.192 64
/27 255.255.255.224 32
/28 255.255.255.240 16
/29 255.255.255.248 8
/30 255.255.255.252 4

Mask table

Mask = /24

0-255

Mask = /25

0-127
128-255

Mask = /26

0-63
64-127
128-191
192-255

Mask = /27

0-31
32-63
64-95
96-127
128-159
160-191
192-223
224-255

Mask = /28

0-15
16-31
32-47
48-63
64-79
80-95
96-111
112-127
128-143
144-159
160-175
176-191
192-207
208-223
224-239
240-255

Mask = /29

 0-7
8-15
16-23
24-31
32-39
40-47
48-55
56-63
64-71
72-79
80-87
88-95
96-103
104-111
112-119
120-127
128-135
136-143
144-151
152-159
160-167
168-175
176-183
184-191
192-199
200-207
208-215
216-223
224-231
232-239
240-247
248-255

Mask = /30

0-3
4-7
8-11
12-15
16-19
20-23
24-27
28-31
32-35
36-39
40-43
44-47
48-51
52-55
56-59
60-63
64-67
68-71
72-75
76-79
80-83
84-87
88-91
92-95
96-99
100-103
104-107
108-111
112-115
116-119
120-123
124-127
128-131
132-135
136-139
140-143
144-147
148-151
152-155
156-159
160-163
164-167
168-171
172-175
176-179
180-183
184-187
188-191
192-195
196-199
200-203
204-207
208-211
212-215
216-219
220-223
224-227
228-231
232-235
236-239
240-243
244-247
248-251
252-255

Netmasks

Netmask              Netmask (binary)                 CIDR     Notes
255.255.255.255  11111111.11111111.11111111.11111111  /32  Host (single addr)
255.255.255.254  11111111.11111111.11111111.11111110  /31  Unuseable
255.255.255.252  11111111.11111111.11111111.11111100  /30    2  useable
255.255.255.248  11111111.11111111.11111111.11111000  /29    6  useable
255.255.255.240  11111111.11111111.11111111.11110000  /28   14  useable
255.255.255.224  11111111.11111111.11111111.11100000  /27   30  useable
255.255.255.192  11111111.11111111.11111111.11000000  /26   62  useable
255.255.255.128  11111111.11111111.11111111.10000000  /25  126  useable
255.255.255.0    11111111.11111111.11111111.00000000  /24 "Class C" 254 useable
255.255.254.0    11111111.11111111.11111110.00000000  /23    2  Class C's
255.255.252.0    11111111.11111111.11111100.00000000  /22    4  Class C's
255.255.248.0    11111111.11111111.11111000.00000000  /21    8  Class C's
255.255.240.0    11111111.11111111.11110000.00000000  /20   16  Class C's
255.255.224.0    11111111.11111111.11100000.00000000  /19   32  Class C's
255.255.192.0    11111111.11111111.11000000.00000000  /18   64  Class C's
255.255.128.0    11111111.11111111.10000000.00000000  /17  128  Class C's
255.255.0.0      11111111.11111111.00000000.00000000  /16  "Class B"

255.254.0.0      11111111.11111110.00000000.00000000  /15    2  Class B's
255.252.0.0      11111111.11111100.00000000.00000000  /14    4  Class B's
255.248.0.0      11111111.11111000.00000000.00000000  /13    8  Class B's
255.240.0.0      11111111.11110000.00000000.00000000  /12   16  Class B's
255.224.0.0      11111111.11100000.00000000.00000000  /11   32  Class B's
255.192.0.0      11111111.11000000.00000000.00000000  /10   64  Class B's
255.128.0.0      11111111.10000000.00000000.00000000  /9   128  Class B's
255.0.0.0        11111111.00000000.00000000.00000000  /8   "Class A"
254.0.0.0        11111110.00000000.00000000.00000000  /7
252.0.0.0        11111100.00000000.00000000.00000000  /6
248.0.0.0        11111000.00000000.00000000.00000000  /5
240.0.0.0        11110000.00000000.00000000.00000000  /4
224.0.0.0        11100000.00000000.00000000.00000000  /3
192.0.0.0        11000000.00000000.00000000.00000000  /2
128.0.0.0        10000000.00000000.00000000.00000000  /1
0.0.0.0          00000000.00000000.00000000.00000000  /0   IP space

kaynak: http://wiki.xtronics.com/index.php/IP_Subnet_Masks