Prefix Length
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... to be assigned/allocated in classless blocks. With this in mind,
assignments will not be made in Class Cs or Bs but by prefix
length. Consequently, an organization that would have been
assigned a Class B in the past will now be assigned a /16 prefix ...
... CIDR blocks of less than /16. Local
IRs/ISPs with a prefix length of /16 or shorter will be responsible
for maintaining all IN-ADDR.ARPA resource records ...