|                   subnet prefix                | interface ID   |
   +------------------------------------------------+----------------+

   Still more sophisticated hosts may be aware of other hierarchical
   boundaries in the unicast address.  Though a very simple router may
   have no knowledge of the internal structure of IPv6 unicast
   addresses, routers will more generally have knowledge of one or more
   of the hierarchical boundaries for the operation of routing
   protocols.  The known boundaries will differ from router to router,
   depending on what positions the router holds in the routing
   hierarchy.

2.5.1 Interface Identifiers

   Interface identifiers in IPv6 unicast addresses are used to identify
   interfaces on a link.  They are required to be unique on that link.
   They may also be unique over a broader scope.  In many cases an
   interface's identifier will be the same as that interface's link-
   layer address.  The same interface identifier may be used on multiple
   interfaces on a single node.

   Note that the use of the same interface identifier on multiple
   interfaces of a single node does not affect the interface
   identifier's global uniqueness or each IPv6 addresses global
   uniqueness created using that interface identifier.

   In a number of the format prefixes (see section 2.4) Interface IDs
   are required to be 64 bits long and to be constructed in IEEE EUI-64
   format [EUI64].  EUI-64 based Interface identifiers may have global
   scope when a global token is available (e.g., IEEE 48bit MAC) or may
   have local scope where a global token is not available (e.g., serial
   links, tunnel end-points, etc.).  It is required that the "u" bit
   (universal/local bit in IEEE EUI-64 terminology) be inverted when
   forming the interface identifier from the EUI-64.  The "u" bit is set
   to one (1) to indicate global scope, and it is set to zero (0) to
   indicate local scope.  The first three octets in binary of an EUI-64
   identifier are as follows:

       0       0 0       1 1       2
      |0       7 8       5 6       3|
      +----+----+----+----+----+----+
      |cccc|ccug|cccc|cccc|cccc|cccc|
      +----+----+----+----+----+----+







 
RFC 2373              IPv6 Addressing Architecture             July 1998


   written in Internet standard bit-order , where "u" is the
   universal/local bit, "g" is the individual/group bit, and "c" are the
   bits of the company_id.  Appendix A: "Creating EUI-64 based Interface
   Identifiers" provides examples on the creation of different EUI-64
   based interface identifiers.

   The motivation for inverting the "u" bit when forming the interface
   identifier is to make it easy for system administrators to hand
   configure local scope identifiers when hardware tokens are not
   available.  This is expected to be case for serial links, tunnel end-
   points, etc.  The alternative would have been for these to be of the
   form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler ::1,
   ::2, etc.

   The use of the universal/local bit in the IEEE EUI-64 identifier is
   to allow development of future technology that can take advantage of
   interface identifiers with global scope.

   The details of forming interface identifiers are defined in the
   appropriate "IPv6 over " specification such as "IPv6 over
   Ethernet" [ETHER], "IPv6 over FDDI" [FDDI], etc.

2.5.2 The Unspecified Address

   The address 0:0:0:0:0:0:0:0 is called the unspecified address.  It
   must never be assigned to any node.  It indicates the absence of an
   address.  One example of its use is in the Source Address field of
   any IPv6 packets sent by an initializing host before it has learned
   its own address.

   The unspecified address must not be used as the destination address
   of IPv6 packets or in IPv6 Routing Headers.

2.5.3 The Loopback Address

   The unicast address 0:0:0:0:0:0:0:1 is called the loopback address.
   It may be used by a node to send an IPv6 packet to itself.  It may
   never be assigned to any physical interface.  It may be thought of as
   being associated with a virtual interface (e.g., the loopback
   interface).

   The loopback address must not be used as the source address in IPv6
   packets that are sent outside of a single node.  An IPv6 packet with
   a destination address of loopback must never be sent outside of a
   single node and must never be forwarded by an IPv6 router.