Quick reference of various subnet masks:

        cidr    mask    nets    usable hosts/net
class-b /16     B.0.0   256*c   65,024
        /17     B.128.0 128*c   32,512
        /18     B.192.0 64*c    16,256
        /19     B.224.0 32*c    8,128
        /20     B.240.0 16*c    4,064
        /21     B.248.0 8*c     2,032
        /22     B.252.0 4*c     1,016
        /23     B.254.0 2*c     508
class-c /24     C.0     1       254
        /25     C.128   2       126
        /26     C.192   4       62
        /27     C.224   8       30
        /28     C.240   16      14
        /29     C.248   32      6
ptp     /30     C.252   64      2

Above, in mask, the following shorthand is used:
        B = 255.255.x.y
        C = 255.255.255.x

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Special subnets, reserved for non-Internet use by
any entity:

10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255