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

2.6.1 Required Anycast Address

   The Subnet-Router anycast address is predefined.  Its 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 2373              IPv6 Addressing Architecture             July 1998


   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 its "home" subnet.

2.7 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)
         B  (unassigned)
         C  (unassigned)




 
RFC 2373              IPv6 Addressing Architecture             July 1998


         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
   independent of the scope value.  For example, if the "NTP servers
   group" is assigned a permanent multicast address with a group ID of