Networking/Subnets
This is a complete rip from http://wiki.xtronics.com/index.php/IP_Subnet_Masks
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.
| 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 |
| /31 | 255.255.255.254 | 2 |
| /32 | 255.255.255.255 | 1 |
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 with in 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
80-95
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 | 2 Class A's |
| 252.0.0.0 | 11111100.00000000.00000000.00000000 | /6 | 4 Class A's |
| 248.0.0.0 | 11111000.00000000.00000000.00000000 | /5 | 8 Class A's |
| 240.0.0.0 | 11110000.00000000.00000000.00000000 | /4 | 16 Class A's |
| 224.0.0.0 | 11100000.00000000.00000000.00000000 | /3 | 32 Class A's |
| 192.0.0.0 | 11000000.00000000.00000000.00000000 | /2 | 64 Class A's |
| 128.0.0.0 | 10000000.00000000.00000000.00000000 | /1 | 128 Class A's |
| 0.0.0.0 | 00000000.00000000.00000000.00000000 | /0 | IP Space |