o An anycast address MUST NOT be assigned to an IPv6 host, that
        is, it may be assigned to an IPv6 router only.


   2.5.1 Required Anycast Address

   The Subnet-Router anycast address is predefined.  It's format is as
   follows:


    |                         n bits                 |   128-n bits   |
    +------------------------------------------------+----------------+
    |                   subnet prefix                | 00000000000000 |
    +------------------------------------------------+----------------+


   The "subnet prefix" in an anycast address is the prefix which
   identifies a specific link.  This anycast address is syntactically
   the same as a unicast address for an interface on the link with the
   interface identifier set to zero.

   Packets sent to the Subnet-Router anycast address will be delivered
   to one router on the subnet.  All routers are required to support the
   Subnet-Router anycast addresses for the subnets which they have
   interfaces.





 
RFC 1884              IPv6 Addressing Architecture         December 1995


   The subnet-router anycast address is intended to be used for
   applications where a node needs to communicate with one of a set of
   routers on a remote subnet.  For example when a mobile host needs to
   communicate with one of the mobile agents on it's "home" subnet.


   2.6 Multicast Addresses

   An IPv6 multicast address is an identifier for a group of nodes.  A
   node may belong to any number of multicast groups.  Multicast
   addresses have the following format:

    |   8    |  4 |  4 |                  112 bits                   |
    +------ -+----+----+---------------------------------------------+
    |11111111|flgs|scop|                  group ID                   |
    +--------+----+----+---------------------------------------------+

        11111111 at the start of the address identifies the address as
        being a multicast address.

                                      +-+-+-+-+
        flgs is a set of 4 flags:     |0|0|0|T|
                                      +-+-+-+-+

             The high-order 3 flags are reserved, and must be
             initialized to 0.

             T = 0 indicates a permanently-assigned ("well-known")
             multicast address, assigned by the global internet
             numbering authority.

             T = 1 indicates a non-permanently-assigned ("transient")
             multicast address.

        scop is a 4-bit multicast scope value used to limit the scope of
        the multicast group.  The values are:

             0  reserved
             1  node-local scope
             2  link-local scope
             3  (unassigned)
             4  (unassigned)
             5  site-local scope
             6  (unassigned)
             7  (unassigned)
             8  organization-local scope
             9  (unassigned)
             A  (unassigned)




 
RFC 1884              IPv6 Addressing Architecture         December 1995


             B  (unassigned)
             C  (unassigned)
             D  (unassigned)
             E  global scope
             F  reserved

        group ID identifies the multicast group, either permanent or
        transient, within the given scope.

   The "meaning" of a permanently-assigned multicast address is