IPv6 - Valid and Preferred Addresses
IPv6 uses stateless autoconfiguration to assign IPv6 addresses to IPv6 hosts without the use of a DHCP server. This is done using IPv6's Neighbor Discovery Protocol with the exchange of router advertisements (RA) and router solicitation (RS) messages.
Within this mechanism, addresses are given particular characteristics. These, as described in RFC 4862 are the following:
- Preferred Address - an address assigned to an interface whose use by upper-layer protocols is unrestricted. Preferred addresses may be used as the source (or destination) address of packets sent from (or to) the interface.
- Deprecated address - An address assigned to an interface whose use is discouraged, but not forbidden. A deprecated address should no longer be used as a source address in new communications, but packets sent from or to deprecated addresses are delivered as expected. A deprecated address may continue to be used as a source address in communications where switching to a preferred address causes hardship to a specific upper-layer activity (e.g., an existing TCP connection).
- Valid address - a preferred or deprecated address. A valid address may appear as the source or destination address of a packet, and the Internet routing system is expected to deliver packets sent to a valid address to their intended recipients.
- Invalid address - an address that is not assigned to any interface. A valid address becomes invalid when its valid lifetime expires. Invalid addresses should not appear as the destination or source address of a packet. In the former case, the Internet routing system will be unable to deliver the packet; in the latter case, the recipient of the packet will be unable to respond to it.
- Preferred Lifetime - the length of time that a valid address is preferred (i.e., the time until deprecation). When the preferred lifetime expires, the address becomes deprecated.
- Valid Lifetime - the length of time an address remains in the valid state (i.e., the time until invalidation). The valid lifetime must be greater than or equal to the preferred lifetime. When the valid lifetime expires, the address becomes invalid.
For more information, also take a look at IPv6 - NDP preferred and valid lifetimes.