IPv6のアドレス体系（アドレスの文字表記）

   There are three conventional forms for representing IPv6 addresses as
   text strings:

     1. The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the
        hexadecimal values of the eight 16-bit pieces of the address.
        Examples:

                   FEDC:BA98:7654:3210:FEDC:BA98:7654:3210

                   1080:0:0:0:8:800:200C:417A

        Note that it is not necessary to write the leading zeros in an
        individual field, but there must be at least one numeral in
        every field (except for the case described in 2.).

     2. Due to the method of allocating certain styles of IPv6
        addresses, it will be common for addresses to contain long
        strings of zero bits.  In order to make writing addresses
        containing zero bits easier a special syntax is available to
        compress the zeros.  The use of "::" indicates multiple groups
        of 16-bits of zeros.  The "::" can only appear once in an
        address.  The "::" can also be used to compress the leading
        and/or trailing zeros in an address.

        For example the following addresses:

             1080:0:0:0:8:800:200C:417A  a unicast address
             FF01:0:0:0:0:0:0:43         a multicast address
             0:0:0:0:0:0:0:1             the loopback address
             0:0:0:0:0:0:0:0             the unspecified addresses

        may be represented as:

             1080::8:800:200C:417A       a unicast address
             FF01::43                    a multicast address
             ::1                         the loopback address
             ::                          the unspecified addresses

     3. An alternative form that is sometimes more convenient when
        dealing with a mixed environment of IPv4 and IPv6 nodes is
        x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values
        of the six high-order 16-bit pieces of the address, and the 'd's
        are the decimal values of the four low-order 8-bit pieces of the
        address (standard IPv4 representation).  Examples:

             0:0:0:0:0:0:13.1.68.3

             0:0:0:0:0:FFFF:129.144.52.38

        or in compressed form:

             ::13.1.68.3

             ::FFFF:129.144.52.38