Reports the preferred lifetime, in seconds, that the client requests for its IPv6 address -- the duration after which the address becomes deprecated but is still usable. This is the value the client asks for in its Solicit/Request message; the server may grant a different (usually shorter) lifetime in its Reply, so an admin comparing this against the address's actual assigned lifetime can spot a server that's overriding client preferences. A client might request 604800 seconds (7 days) as its preferred lifetime, though the server's actual grant could be shorter.