IPv6のアドレス体系（IPv6アドレス）

   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   superseded by multicast addresses.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.