2.7.2 Assignment of New IPv6 Multicast Addresses

   The current approach [ETHER] to map IPv6 multicast addresses into
   IEEE 802 MAC addresses takes the low order 32 bits of the IPv6
   multicast address and uses it to create a MAC address.  Note that
   Token Ring networks are handled differently.  This is defined in
   [TOKEN].  Group ID's less than or equal to 32 bits will generate
   unique MAC addresses.  Due to this new IPv6 multicast addresses
   should be assigned so that the group identifier is always in the low
   order 32 bits as shown in the following:

   |   8    |  4 |  4 |          80 bits          |     32 bits     |
   +------ -+----+----+---------------------------+-----------------+
   |11111111|flgs|scop|   reserved must be zero   |    group ID     |
   +--------+----+----+---------------------------+-----------------+

   While this limits the number of permanent IPv6 multicast groups to
   2^32 this is unlikely to be a limitation in the future.  If it
   becomes necessary to exceed this limit in the future multicast will
   still work but the processing will be sightly slower.

   Additional IPv6 multicast addresses are defined and registered by the
   IANA [MASGN].

2.8 A Node's Required Addresses

   A host is required to recognize the following addresses as
   identifying itself:

      o Its Link-Local Address for each interface
      o Assigned Unicast Addresses
      o Loopback Address
      o All-Nodes Multicast Addresses
      o Solicited-Node Multicast Address for each of its assigned
        unicast and anycast addresses
      o Multicast Addresses of all other groups to which the host
        belongs.

   A router is required to recognize all addresses that a host is
   required to recognize, plus the following addresses as identifying
   itself:

      o The Subnet-Router anycast addresses for the interfaces it is
        configured to act as a router on.
      o All other Anycast addresses with which the router has been
        configured.
      o All-Routers Multicast Addresses





 
RFC 2373              IPv6 Addressing Architecture             July 1998


      o Multicast Addresses of all other groups to which the router
        belongs.

   The only address prefixes which should be predefined in an
   implementation are the:

      o Unspecified Address
      o Loopback Address
      o Multicast Prefix (FF)
      o Local-Use Prefixes (Link-Local and Site-Local)
      o Pre-Defined Multicast Addresses
      o IPv4-Compatible Prefixes

   Implementations should assume all other addresses are unicast unless
   specifically configured (e.g., anycast addresses).

3. Security Considerations

   IPv6 addressing documents do not have any direct impact on Internet
   infrastructure security.  Authentication of IPv6 packets is defined
   in [AUTH].