RFC 1884              IPv6 Addressing Architecture         December 1995


1.0 INTRODUCTION

   This specification defines the addressing architecture of the IP
   Version 6 protocol.  It includes a detailed description of the
   currently defined address formats for IPv6 [IPV6].

   The editors would like to acknowledge the contributions of Paul
   Francis, Jim Bound, Brian Carpenter, Deborah Estrin, Peter Ford, Bob
   Gilligan, Christian Huitema, Tony Li, Greg Minshall, Erik Nordmark,
   Yakov Rekhter, Bill Simpson, and Sue Thomson.

2.0 IPv6 ADDRESSING

   IPv6 addresses are 128-bit identifiers for interfaces and sets of
   interfaces.  There are three types of addresses:


       Unicast:   An identifier for a single interface.  A packet sent
                  to a unicast address is delivered to the interface
                  identified by that address.

       Anycast:   An identifier for a set of interfaces (typically
                  belonging to different nodes).  A packet sent to an
                  anycast address is delivered to one of the interfaces
                  identified by that address (the "nearest" one,
                  according to the routing protocols' measure of
                  distance).

       Multicast: An identifier for a set of interfaces (typically
                  belonging to different nodes).  A packet sent to a
                  multicast address is delivered to all interfaces
                  identified by that address.

   There are no broadcast addresses in IPv6, their function being
   superseded by multicast addresses.

   In this document, fields in addresses are given a specific name, for
   example "subscriber".  When this name is used with the term "ID" for
   identifier after the name (e.g., "subscriber ID"), it refers to the
   contents of the named field.  When it is used with the term "prefix"
   (e.g., "subscriber prefix") it refers to all of the address up to and
   including this field.

   In IPv6, all zeros and all ones are legal values for any field,
   unless specifically excluded.  Specifically, prefixes may contain
   zero-valued fields or end in zeros.






 
RFC 1884              IPv6 Addressing Architecture         December 1995


   2.1 Addressing Model

   IPv6 Addresses of all types are assigned to interfaces, not nodes.
   Since each interface belongs to a single node, any of that node's
   interfaces' unicast addresses may be used as an identifier for the
   node.

   An IPv6 unicast address refers to a single interface.  A single
   interface may be assigned multiple IPv6 addresses of any type
   (unicast, anycast, and multicast).  There are two exceptions to this
   model.  These are:

     1) A single address may be assigned to multiple physical interfaces
        if the implementation treats the multiple physical interfaces as
        one interface when presenting it to the internet layer.  This is
        useful for load-sharing over multiple physical interfaces.

     2) Routers may have unnumbered interfaces (i.e., no IPv6 address
        assigned to the interface) on point-to-point links to eliminate
        the necessity to manually configure and advertise the addresses.
        Addresses are not needed for point-to-point interfaces on
        routers if those interfaces are not to be used as the origins or
        destinations of any IPv6 datagrams.

   IPv6 continues the IPv4 model that a subnet is associated with one
   link.  Multiple subnets may be assigned to the same link.