In the context of IPv6 addresses, valid_lft (valid lifetime) and preferred_lft (preferred lifetime) are parameters that define the usability of an IPv6 address assigned to an interface.
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valid_lft (Valid Lifetime):
- This parameter indicates the total time period for which the IPv6 address is considered valid. After this time expires, the address can no longer be used for communication, and the device must either renew the address or stop using it.
- The valid lifetime is typically set by the router that provides the address through Stateless Address Autoconfiguration (SLAAC) or DHCPv6.
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preferred_lft (Preferred Lifetime):
- This parameter specifies the time period during which the IPv6 address is preferred for use. While the address is within its preferred lifetime, it is the address that should be used for outgoing connections.
- Once the preferred lifetime expires, the address is still valid (as long as the valid lifetime has not expired), but it is no longer preferred. The device may choose to use other addresses that are still within their preferred lifetime for new connections.
In summary, valid_lft determines how long an address can be used, while preferred_lft indicates how long the address is preferred for use.
