Using a subnet prefix length other than a /64 will break many features of IPv6, including Neighbor Discovery (ND), Secure Neighbor Discovery (SEND) [RFC3971], privacy extensions [RFC4941], parts of Mobile IPv6 [RFC4866], Protocol Independent Multicast - Sparse Mode (PIM-SM) with Embedded-RP [RFC3956], and Site Multihoming by IPv6 Intermediation (SHIM6) [SHIM6], among others. A number of other features currently in development, or being proposed, also rely on /64 subnet prefixes. Nevertheless, many IPv6 implementations do not prevent the administrator from configuring a subnet prefix length shorter or longer than 64 bits. Using subnet prefixes shorter than /64 would rarely be useful; see Appendix B.1 for discussion. However, some network administrators have used prefixes longer than /64 for links connecting routers, usually just two routers on a point-to-point link. On links where all the addresses are assigned by manual configuration, and all nodes on the link are routers (not end hosts) that are known by the network, administrators do not need any of the IPv6 features that rely on /64 subnet prefixes, this can work. Using subnet prefixes longer than /64 is not recommended for general use, and using them for links containing end hosts would be an especially bad idea, as it is difficult to predict what IPv6 features the hosts will use in the future.