# File lib/ipaddress/ipv4.rb, line 957
    def self.parse_classful(ip)
      if IPAddress.valid_ipv4?(ip)
        address = ip.strip
      else
        raise ArgumentError, "Invalid IP #{ip.inspect}"
      end
      prefix = CLASSFUL.find{|h,k| h === ("%.8b" % address.to_i)}.last
      self.new "#{address}/#{prefix}"
    end