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IP Subnet Masks

The first address of a subnet block (all 0s) is called the network address or network ID. The last address (all 1s) is the broadcast address of the network. Typically the network address +1 or the broadcast address -1 is the gateway to the internet. This leaves us with total number of IP numbers -3 left over for host address within a sub net block. That's why you either get 1 IP (4-3= 1) or if you ask for one more you get 5 (8-3=5).

Here is an example:

192.168.1.0/25 would include all address between 192.168.1.0 and 192.168.1.127

while 192.168.1.128/25 would include 192.168.1.128 and 192.168.1.255

Below is a mask table that makes it easy to look up the mask for a group of IP addresses.

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
80-111
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 2  usable (RFC 3021)
255.255.255.252  11111111.11111111.11111111.11111100  /30 4  usable
255.255.255.248  11111111.11111111.11111111.11111000  /29 6  usable
255.255.255.240  11111111.11111111.11111111.11110000  /28 14  usable
255.255.255.224  11111111.11111111.11111111.11100000  /27 30  usable
255.255.255.192  11111111.11111111.11111111.11000000  /26 62  usable
255.255.255.128  11111111.11111111.11111111.10000000  /25  126  usable
255.255.255.0 11111111.11111111.11111111.00000000  /24  254  usable
255.255.254.0 11111111.11111111.11111110.00000000  /23 2  Class C's + 2
255.255.252.0 11111111.11111111.11111100.00000000  /22 4  Class C's + 6
255.255.248.0 11111111.11111111.11111000.00000000  /21 8  Class C's + 14
255.255.240.0 11111111.11111111.11110000.00000000  /20 16  Class C's + 30
255.255.224.0 11111111.11111111.11100000.00000000  /19 32  Class C's + 62
255.255.192.0 11111111.11111111.11000000.00000000  /18 64  Class C's + 126
255.255.128.0 11111111.11111111.10000000.00000000  /17  128  Class C's + 254 / 129 
255.255.0.0 11111111.11111111.00000000.00000000  /16  
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 
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


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