IPv6 addressing architecture allows multiple unicast addresses to be assigned to interfaces. These addresses might have different reachability scopes (link-local or global). These addresses can also be preferred or deprecated. Privacy considerations have introduced the concepts of public addresses and temporary addresses. The mobility architecture introduces home addresses and care-of addresses. In addition, multihoming situations result in more addresses per node. For example, a node can have multiple interfaces, some of them tunnels or virtual interfaces, or a site can have multiple ISP attachments with a global prefix per ISP.

The end result is that IPv6 implementations are often faced with multiple possible source and destination addresses when initiating communication. It is preferred to have default algorithms, common across all implementations, for selecting source and destination addresses so that developers and administrators can reason about and predict the behavior of their systems.

Furthermore, dual-mode stack implementations, which support both IPv6 and IPv4, very often need to choose between IPv6 and IPv4 when initiating communication. For example, DNS name resolution might yield both IPv6 and IPv4 addresses with the network protocol stack having both IPv6 and IPv4 source addresses available. In these cases, a policy that always prefers IPv6 or always prefers IPv4 might produce poor results. For example, if a DNS name resolves to a global IPv6 address and a global IPv4 address. If the node has assigned a global IPv6 address and a 169.254/16 autoconfigured IPv4 address, then IPv6 is the best choice for communication because the global address has a similar scope; therefore, a better chance of success. But if the node has assigned only a link-local IPv6 address and a global IPv4 address, then IPv4 is the best choice for communication because the scope more closely matches the scope of the destination to which you are communicating. The destination address selection algorithm solves this with a unified procedure for choosing among both IPv6 and IPv4 addresses.

Source address selection and destination address selection are discussed separately, but using a common framework enables the two algorithms together to yield useful results. The algorithms attempt to choose source and destination addresses of appropriate scope and configuration status (preferred or deprecated).