The stateless approach is used when a site is not concerned with the exact addresses that hosts use. However, the addresses must be unique and must be properly routable. The stateful approach is used when a site requires more precise control over exact address assignments. Stateful and stateless address autoconfiguration can be used simultaneously. The site administrator specifies which type of autoconfiguration to use through the setting of appropriate fields in router advertisement messages.
IPv6 addresses are leased to an interface for a fixed (possibly infinite) length of time. Each address has an associated lifetime that indicates how long the address is bound to an interface. When a lifetime expires, the binding (and address) become invalid and the address can be reassigned to another interface elsewhere. To handle the expiration of address bindings gracefully, an address experiences two distinct phases while the address is assigned to an interface. Initially, an address is preferred, meaning that its use in arbitrary communication is unrestricted. Later, an address becomes deprecated in anticipation that its current interface binding becomes invalid. When the address is in a deprecated state, the use of the address is discouraged, but not strictly forbidden. New communication (for example, the opening of a new TCP connection) should use a preferred address when possible. A deprecated address should be used only by applications that have been using the address. Applications that cannot switch to another address without a service disruption can use a deprecated address.