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

=================================================
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

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