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. This is the full form. For example,
Note that it is not necessary to write the leading zeros in an individual field. However, there must be at least one numeral in every field, except as described below.
Due to some methods 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,
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 standard IPv4 representation address, for example,
where "::FFFF:d.d.d.d" and "::d.d.d.d" are, respectively, the general forms of an IPv4-mapped IPv6 address and an IPv4-compatible IPv6 address. Note that the IPv4 portion must be in the "d.d.d.d" form. The following forms are invalid:
::FFFF:d.d.d ::FFFF:d.d ::d.d.d ::d.d
The following form:
is valid, however it is an unconventional representation of the IPv4-compatible IPv6 address,
while "::d" corresponds to the general IPv6 address "0:0:0:0:0:0:0:d".
For methods that return a textual representation as output value, the full form is used. Inet6Address will return the full form because it is unambiguous when used in combination with other textual data.
IPv4-mapped address Of the form::ffff:w.x.y.z, this IPv6 address is used to represent an IPv4 address. It allows the native program to use the same address data structure and also the same socket when communicating with both IPv4 and IPv6 nodes.
In InetAddress and Inet6Address, it is used for internal representation; it has no functional role. Java will never return an IPv4-mapped address. These classes can take an IPv4-mapped address as input, both in byte array and text representation. However, it will be converted into an IPv4 address.
The textual representation of IPv6 addresses as described above can be extended to specify IPv6 scoped addresses. This extension to the basic addressing architecture is described in [draft-ietf-ipngwg-scoping-arch-04.txt].
Because link-local and site-local addresses are non-global, it is possible that different hosts may have the same destination address and may be reachable through different interfaces on the same originating system. In this case, the originating system is said to be connected to multiple zones of the same scope. In order to disambiguate which is the intended destination zone, it is possible to append a zone identifier (or scope_id) to an IPv6 address.
The general format for specifying the scope_id is the following:
The IPv6-address is a literal IPv6 address as described above. The scope_id refers to an interface on the local system, and it can be specified in two ways.
Note also, that the numeric scope_id can be retrieved from Inet6Address instances returned from the NetworkInterface class. This can be used to find out the current scope ids configured on the system. @since 1.4