DNS Short Hand For Netmasks

The number in the "/xx" shorthand stands for the number of bits (technically, bits set to one) in the subnet mask. The convention is always to start at the left end of the 32-bit subnet mask. The table below shows the correspondence between the "/xx" notation and the actual numeric representation.

Subnet Mask # of Addresses
/1 128.0.0.0 2.1 billion
/2 192.0.0.0 1 billion
/3 224.0.0.0 536 million
/4 240.0.0.0 268 million
/5 248.0.0.0 134 million
/6 252.0.0.0 67 million
/7 254.0.0.0 34 million
/8 255.0.0.0 17 million (Class A)
/9 255.128.0.0 8.4 million
/10 255.192.0.0 4.2 million
/11 255.224.0.0 2.1 million
/12 255.240.0.0 1 million
/13 255.248.0.0 524 thousand
/14 255.252.0.0 262 thousand
/15 255.254.0.0 131 thousand
/16 255.255.0.0 65,534 (Class B)
/17 255.255.128.0 32,766
/18 255.255.192.0 16,382
/19 255.255.224.0 8,190
/20 255.255.240.0 4,094
/21 255.255.248.0 2,046
/22 255.255.252.0 1,022
/23 255.255.254.0 510
/24 255.255.255.0 254 (Class C)
/25 255.255.255.128 126
/26 255.255.255.192 62
/27 255.255.255.224 30 (254-224)
/28 255.255.255.240 14 (254-240)
/29 255.255.255.248 6 (254-248 or 2*2*2-2)
/30 255.255.255.252 2 (254-252 or 2*2-2)
/31 255.255.255.254 RFC 3021
/32 255.255.255.255 Loopback address