to one router on the subnet.  All routers are required to support the
   Subnet-Router anycast addresses for the subnets to which they have
   interfaces.

   The subnet-router anycast address is intended to be used for
   applications where a node needs to communicate with any one of the
   set of routers.

2.7 Multicast Addresses

   An IPv6 multicast address is an identifier for a group of interfaces
   (typically on different nodes).  An interface may belong to any
   number of multicast groups.  Multicast addresses have the following
   format:

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

         binary 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|
                                       +-+-+-+-+




 
RFC 3513              IPv6 Addressing Architecture            April 2003


            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 Internet Assigned Number
            Authority (IANA).

            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  interface-local scope
            2  link-local scope
            3  reserved
            4  admin-local scope
            5  site-local scope
            6  (unassigned)
            7  (unassigned)
            8  organization-local scope
            9  (unassigned)
            A  (unassigned)
            B  (unassigned)
            C  (unassigned)
            D  (unassigned)
            E  global scope
            F  reserved

            interface-local scope spans only a single interface on a
            node, and is useful only for loopback transmission of
            multicast.

            link-local and site-local multicast scopes span the same
            topological regions as the corresponding unicast scopes.

            admin-local scope is the smallest scope that must be
            administratively configured, i.e., not automatically derived
            from physical connectivity or other, non- multicast-related
            configuration.

            organization-local scope is intended to span multiple sites
            belonging to a single organization.

            scopes labeled "(unassigned)" are available for
            administrators to define additional multicast regions.





 
RFC 3513              IPv6 Addressing Architecture            April 2003


         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
   101 (hex), then:

      FF01:0:0:0:0:0:0:101 means all NTP servers on the same interface
      (i.e., the same node) as the sender.