def nth_subnet(prefix_len,index)
			count = self.subnet_count(prefix_len)
			if (count == 0 || index >= count)
				return nil
			end
			sub0 = IPv6Net.new(self.network, Mask128.new(prefix_len))
			return sub0.nth_next_sib(index)
		end
def nth_subnet(prefix_len,index)
			count = self.subnet_count(prefix_len)
			if (count == 0 || index >= count)
				return nil
			end
			sub0 = IPv6Net.new(self.network, Mask128.new(prefix_len))
			return sub0.nth_next_sib(index)
		end
nth_subnet returns the subnet IPv6Net at the given index.
 The number of subnets may be determined with the subnet_count() method.
 If the range is exceeded  or an invalid prefix_len is provided then return nil.