# File lib/cidr.rb, line 585
    def arpa()

        base = self.ip()
        netmask = self.bits()

        if (@version == 4)
            net = base.split('.')

            if (netmask)
                while (netmask < 32)
                    net.pop
                    netmask = netmask + 8
                end
            end

            arpa = net.reverse.join('.')
            arpa << ".in-addr.arpa."

        elsif (@version == 6)
            fields = base.split(':')
            net = []
            fields.each do |field|
                (field.split("")).each do |x|
                    net.push(x)
                end
            end

            if (netmask)
                while (netmask < 128)
                    net.pop
                    netmask = netmask + 4
                end
            end

            arpa = net.reverse.join('.')
            arpa << ".ip6.arpa."

        end

        return(arpa)
    end