Google Google Google NetDEF, Inc. This document defines a "P" flag in the Prefix Information Option (PIO) of IPv6 Router Advertisements (RAs). The flag is used to indicate that the network prefers that clients use the RFC9663 deployment model instead of using individual adresses in the on-link prefix assigned using Stateless Address Autoconfiguration (SLAAC) or DHCPv6 address assignment. This document updates RFC4862 to indicate that the Autonomous flag in a PIO needs to be ignored if the PIO has the P flag set. It also updates RFC4861 to specify that the P flag indicates DHCPv6 Prefix Delegation support for clients.