Subnet mask values and figure out what they mean

Here are the charts, followed by some explanations of what they mean.

CIDR SUBNET MASK WILDCARD MASK # OF IP ADDRESSES # OF USABLE IP ADDRESSES
/32 255.255.255.255 0.0.0.0 1 1
/31 255.255.255.254 0.0.0.1 2 2*
/30 255.255.255.252 0.0.0.3 4 2
/29 255.255.255.248 0.0.0.7 8 6
/28 255.255.255.240 0.0.0.15 16 14
/27 255.255.255.224 0.0.0.31 32 30
/26 255.255.255.192 0.0.0.63 64 62
/25 255.255.255.128 0.0.0.127 128 126
/24 255.255.255.0 0.0.0.255 256 254
/23 255.255.254.0 0.0.1.255 512 510
/22 255.255.252.0 0.0.3.255 1,024 1,022
/21 255.255.248.0 0.0.7.255 2,048 2,046
/20 255.255.240.0 0.0.15.255 4,096 4,094
/19 255.255.224.0 0.0.31.255 8,192 8,190
/18 255.255.192.0 0.0.63.255 16,384 16,382
/17 255.255.128.0 0.0.127.255 32,768 32,766
/16 255.255.0.0 0.0.255.255 65,536 65,534
/15 255.254.0.0 0.1.255.255 131,072 131,070
/14 255.252.0.0 0.3.255.255 262,144 262,142
/13 255.248.0.0 0.7.255.255 524,288 524,286
/12 255.240.0.0 0.15.255.255 1,048,576 1,048,574
/11 255.224.0.0 0.31.255.255 2,097,152 2,097,150
/10 255.192.0.0 0.63.255.255 4,194,304 4,194,302
/9 255.128.0.0 0.127.255.255 8,388,608 8,388,606
/8 255.0.0.0 0.255.255.255 16,777,216 16,777,214
/7 254.0.0.0 1.255.255.255 33,554,432 33,554,430
/6 252.0.0.0 3.255.255.255 67,108,864 67,108,862
/5 248.0.0.0 7.255.255.255 134,217,728 134,217,726
/4 240.0.0.0 15.255.255.255 268,435,456 268,435,454
/3 224.0.0.0 31.255.255.255 536,870,912 536,870,910
/2 192.0.0.0 63.255.255.255 1,073,741,824 1,073,741,822
/1 128.0.0.0 127.255.255.255 2,147,483,648 2,147,483,646
/0 0.0.0.0 255.255.255.255 4,294,967,296 4,294,967,294

* /31 is a special case detailed in RFC 3021 where networks with this type of subnet mask can assign two IP addresses as a point-to-point link.

And here’s a table of the decimal to binary conversions for subnet mask and wildcard octets:

SUBNET MASK WILDCARD
0 00000000 255 11111111
128 10000000 127 01111111
192 11000000 63 00111111
224 11100000 31 00011111
240 11110000 15 00001111
248 11111000 7 00000111
252 11111100 3 00000011
254 11111110 1 00000001
255 11111111 0 00000000

Note that the wildcard is just the inverse of the subnet mask.

 

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