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 within a subnet
   prefix.  It is recommended that the same interface identifier not be
   assigned to different nodes on a link.  They may also be unique over
   a broader scope.  In some cases an interface's identifier will be
   derived directly from that interface's link-layer address.  The same
   interface identifier may be used on multiple interfaces on a single
   node, as long as they are attached to different subnets.

   Note that the uniqueness of interface identifiers is independent of
   the uniqueness of IPv6 addresses.  For example, a global unicast
   address may be created with a non-global scope interface identifier
   and a site-local address may be created with a global scope interface
   identifier.

   For all unicast addresses, except those that start with binary value
   000, Interface IDs are required to be 64 bits long and to be
   constructed in Modified EUI-64 format.

   Modified EUI-64 format based Interface identifiers may have global
   scope when derived from a global token (e.g., IEEE 802 48-bit MAC or
   IEEE EUI-64 identifiers [EUI64]) or may have local scope where a
   global token is not available (e.g., serial links, tunnel end-points,
   etc.) or where global tokens are undesirable (e.g., temporary tokens
   for privacy [PRIV]).

   Modified EUI-64 format interface identifiers are formed by inverting
   the "u" bit (universal/local bit in IEEE EUI-64 terminology) when
   forming the interface identifier from IEEE EUI-64 identifiers.  In
   the resulting Modified EUI-64 format 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 IEEE EUI-64 identifier
   are as follows:

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

   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 Modified EUI-64 format




 
RFC 3513              IPv6 Addressing Architecture            April 2003


   Interface Identifiers" provides examples on the creation of Modified
   EUI-64 format based interface identifiers.

   The motivation for inverting the "u" bit when forming an interface
   identifier is to make it easy for system administrators to hand
   configure non-global 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 Modified EUI-64 format
   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.  An IPv6 packet with a
   source address of unspecified must never be forwarded by an IPv6
   router.

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 is treated as
   having link-local scope, and may be thought of as the link-local
   unicast address of a virtual interface (typically called "the
   loopback interface") to an imaginary link that goes nowhere.

   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.  A packet
   received on an interface with destination address of loopback must be