Energy Sciences Network Jisc Tachyon Dynamics This document draws on several years of operational experience to update RFC 6724, defining the concept of "known-local" ULA prefixes that enable ULA-to-ULA communications within fd00::/8 to become preferred over both IPv4-IPv4 and GUA-to-GUA for local use. The document defines the means by which nodes can both identify and insert such prefixes into their address selection policy table. It also clarifies the mandatory, unconditional requirement for support for Rule 5.5 and demotes the preference for 6to4 addresses. These changes to default behavior improve supportability of common use cases, including automatic / unmanaged scenarios, and makes preference for IPv6 over IPv4 consistent in local site networks for both ULA and GUA prefixes. It is recognized that some less common deployment scenarios may require explicit configuration or custom changes to achieve desired operational parameters.