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Subnet Mask Table
Bits in Mask Subnet Mask Hosts Wildcard Mask
8 255.0.0.0 16777214 0.255.255.255
9 255.128.0.0 8388606 0.127.255.255
10 255.192.0.0 4194302 0.63.255.255
11 255.224.0.0 2097150 0.31.255.255
12 255.240.0.0 1048574 0.15.255.255
13 255.248.0.0 524286 0.7.255.255
14 255.252.0.0 262142 0.3.255.255
15 255.254.0.0 131070 0.1.255.255
16 255.255.0.0 65534 0.0.255.255
17 255.255.128.0 32766 0.0.127.255
18 255.255.192.0 16382 0.0.63.255
19 255.255.224.0 8190 0.0.31.255
20 255.255.240.0 4094 0.0.15.255
21 255.255.248.0 2046 0.0.7.255
22 255.255.252.0 1022 0.0.3.255
23 255.255.254.0 510 0.0.1.255
24 255.255.255.0 254 0.0.0.255
25 255.255.255.128 126 0.0.0.127
26 255.255.255.192 62 0.0.0.63
27 255.255.255.224 30 0.0.0.31
28 255.255.255.240 14 0.0.0.15
29 255.255.255.248 6 0.0.0.7
30 255.255.255.252 2 0.0.0.3
31 255.255.255.254 2* 0.0.0.1

Number of hosts on a /31 subnet:
Generally, within a subnet, two host addresses - all-zeros and one all-ones are reserved as network address and broadcast, respectively. The same approach applied to a /31 subnet with only two possible addresses in the host identifier would leave no usable addresses for host assignment. RFC3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links allows assignment of the all-zeros and one all-ones IP addresses to hosts on point-to-point links.

See Also
Private IP Addresses...
IP Subnet Calculator...