3. Address Properties

In the rules given in later sections, addresses of different types (e.g., IPv4, IPv6, multicast, and unicast) are compared against each other. Some of these address types have properties that aren't directly comparable to each other. For example, IPv6 unicast addresses can be "preferred" or "deprecated" [RFC4862], while IPv4 addresses have no such notion. To compare such addresses using the ordering rules (e.g., to use "preferred" addresses in preference to "deprecated" addresses), the following mappings are defined.