are Link-Local and Site-Local.  The Link-Local is for use on a single
   link and the Site-Local is for use in a single site.  Link-Local
   addresses have the following format:

   |   10     |
   |  bits    |        54 bits          |          64 bits           |
   +----------+-------------------------+----------------------------+
   |1111111010|           0             |       interface ID         |
   +----------+-------------------------+----------------------------+

   Link-Local addresses are designed to be used for addressing on a
   single link for purposes such as auto-address configuration, neighbor
   discovery, or when no routers are present.





 
RFC 2373              IPv6 Addressing Architecture             July 1998


   Routers must not forward any packets with link-local source or
   destination addresses to other links.

   Site-Local addresses have the following format:

   |   10     |
   |  bits    |   38 bits   |  16 bits  |         64 bits            |
   +----------+-------------+-----------+----------------------------+
   |1111111011|    0        | subnet ID |       interface ID         |
   +----------+-------------+-----------+----------------------------+

   Site-Local addresses are designed to be used for addressing inside of
   a site without the need for a global prefix.

   Routers must not forward any packets with site-local source or
   destination addresses outside of the site.

2.6 Anycast Addresses

   An IPv6 anycast address is an address that is assigned to more than
   one interface (typically belonging to different nodes), with the
   property that a packet sent to an anycast address is routed to the
   "nearest" interface having that address, according to the routing
   protocols' measure of distance.

   Anycast addresses are allocated from the unicast address space, using
   any of the defined unicast address formats.  Thus, anycast addresses
   are syntactically indistinguishable from unicast addresses.  When a
   unicast address is assigned to more than one interface, thus turning
   it into an anycast address, the nodes to which the address is
   assigned must be explicitly configured to know that it is an anycast
   address.

   For any assigned anycast address, there is a longest address prefix P
   that identifies the topological region in which all interfaces
   belonging to that anycast address reside.  Within the region
   identified by P, each member of the anycast set must be advertised as
   a separate entry in the routing system (commonly referred to as a
   "host route"); outside the region identified by P, the anycast
   address may be aggregated into the routing advertisement for prefix
   P.

   Note that in, the worst case, the prefix P of an anycast set may be
   the null prefix, i.e., the members of the set may have no topological
   locality.  In that case, the anycast address must be advertised as a
   separate routing entry throughout the entire internet, which presents






 
RFC 2373              IPv6 Addressing Architecture             July 1998


   a severe scaling limit on how many such "global" anycast sets may be
   supported.  Therefore, it is expected that support for global anycast
   sets may be unavailable or very restricted.

   One expected use of anycast addresses is to identify the set of
   routers belonging to an organization providing internet service.
   Such addresses could be used as intermediate addresses in an IPv6
   Routing header, to cause a packet to be delivered via a particular
   aggregation or sequence of aggregations.  Some other possible uses
   are to identify the set of routers attached to a particular subnet,
   or the set of routers providing entry into a particular routing
   domain.

   There is little experience with widespread, arbitrary use of internet
   anycast addresses, and some known complications and hazards when
   using them in their full generality [ANYCST].  Until more experience
   has been gained and solutions agreed upon for those problems, the
   following restrictions are imposed on IPv6 anycast addresses:

      o An anycast address must not be used as the source address of an
        IPv6 packet.