code: def add_ipv6addr(self, ipv6addr):
          
          for addr in self.ipv6addrs:
              if ((isinstance(addr, dict) and addr['ipv6addr'] == ipv6addr) or
                  (isinstance(addr, HostIPv4) and addr.ipv6addr == ipv6addr)):
                  raise ValueError('Already exists')
          self.ipv6addrs.append({'ipv6addr': ipv6addr})
docs: Add an IPv6 address to the host.
  
          :param str ipv6addr: The IP address to add.
          :raises: ValueError