IPv6 addresses are "rented" to an interface for a fixed time, probably infinite.

When this "time of life" expires, the link between the interface and the address is invalidated, and the address can be reallocated to other interface in wherever Internet point.

For the suitable management of addresses expiration time, an address goes through two stages while is assigned to an interface:

a) At first, an address is "preferred", so its use in any communication is no restricted.

b) After that, an address becomes into "deprecated", bringing forward to the fact that its link with the currently interface will be invalidated.

While it is in "deprecated" state, the use of the address is advised against, although it is no forbidden. However, when it was possible, any new communication (for example, the opening of a new TCP connection) must use a "preferred" address. A "deprecated" address should only be used by applications that already used it and it is difficult to change them to other addresses without causing a service interruption.