IPAddr#prefix

# File tmp/rubies/ruby-3.0.5/lib/ipaddr.rb, line 427
def prefix
  case @family
  when Socket::AF_INET
    n = IN4MASK ^ @mask_addr
    i = 32
  when Socket::AF_INET6
    n = IN6MASK ^ @mask_addr
    i = 128
  else
    raise AddressFamilyError, "unsupported address family"
  end
  while n.positive?
    n >>= 1
    i -= 1
  end
  i
end