This document updates the default address selection algorithm for
Internet Protocol Version 6 (IPv6), originally specified in RFC 6724,
based on accumulated operational experience. It introduces the
concept of "known-local" Unique Local Address (ULA) prefixes within
the fd00::/8 block and specifies that ULA-to-ULA communications using
such prefixes should be preferred over both IPv4-to-IPv4 and GUA-to-
GUA (Global Unicast Address) communications in local use scenarios.
The document defines mechanisms for nodes to identify and incorporate
known-local prefixes into their address selection policy tables. It
introduces a requirement to implement Rule 5.5 of RFC 6724 and
reduces the default precedence for 6to4 addresses. These updates
enhance the supportability of typical deployment environments,
including automatic and unmanaged configurations, and promote
consistent IPv6-over-IPv4 precedence behavior for both ULA and GUA
within local networks. The document acknowledges that certain
atypical deployment models may require explicit configuration to
achieve intended operational outcomes.
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