# File lib/net/dns/rr/classes.rb, line 48
        def self.to_str(cls)
          case cls
          when Fixnum
            if Classes.invert.has_key? cls
              return Classes.invert[cls]
            else
              raise ClassArgumentError, "Unknown class number #{cls}"
            end
          else
            raise ClassArgumentError, "Wrong cls class: #{cls.class}"
          end
        end