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IP Subnet Masks

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[edit] IP-Subnet-Mask numbers

IP mask numbers are used to divide internet addresses into blocks called subnets. The mask number represents the number of 1s in the binary of the address that is 'masked" against the address so that it ignores the last bits which are for the group of IP addresses in the masked address.

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. The 'slash' notation (ie /24) is known as CIDR format, while the more conventional 255.255.255.0 notation is considered a subnet mask.

[edit] What is a zero subnet? How has it Changed?

Under old IP subnetting rules, the all 0’s subnet was reserved for the network, and the all 1’s subnet was reserved for the broadcast. Over time, it was found that the all 0’s subnet wasn’t really used and, if it could be handed out as a useable network, many IP addresses could be changed.

An example of an IP address that is using a zero subnet is 10.1.0.1 with a subnet mask of 255.255.255.0. Earlier this was a kind of network id but today, this IP address is perfectly legal when it comes to subnetting. Thus, if I had an IP address of 10.1.0.0 with a 255.255.0.0 subnet mask is the network ID and if we subnet it, we could actually get 255 valid networks out of it by using the 0 subnet.

Net bits

Subnet mask total-addresses per subnet
/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

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).

[edit] 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.

[edit] Mask table

[edit] Mask = /24

0-255

[edit] Mask = /25

0-127
128-255

[edit] Mask = /26

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

[edit] Mask = /27

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

[edit] 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

[edit] 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

[edit] 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

[edit] 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  Unusable
255.255.255.252  11111111.11111111.11111111.11111100  /30    2  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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