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

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).

[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  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