5. Text Representation of Special Addresses

5. Text Representation of Special Addresses

Addresses such as IPv4-Mapped IPv6 addresses, ISATAP [RFC5214], and IPv4-translatable addresses [ADDR-FORMAT] have IPv4 addresses embedded in the low-order 32 bits of the address. These addresses have a special representation that may mix hexadecimal and dot decimal notations. The decimal notation may be used only for the last 32 bits of the address. For these addresses, mixed notation is RECOMMENDED if the following condition is met: the address can be distinguished as having IPv4 addresses embedded in the lower 32 bits solely from the address field through the use of a well-known prefix. Such prefixes are defined in [RFC4291] and [RFC2765] at the time of this writing. If it is known by some external method that a given prefix is used to embed IPv4, it MAY be represented as mixed notation. Tools that provide options to specify prefixes that are (or are not) to be represented as mixed notation may be useful.

There is a trade-off here where a recommendation to achieve an exact match in a search (no dot decimals whatsoever) and a recommendation to help the readability of an address (dot decimal whenever possible) does not result in the same solution. The above recommendation is aimed at fixing the representation as much as possible while leaving the opportunity for future well-known prefixes to be represented in a human-friendly manner as tools adjust to newly assigned prefixes.

The text representation method noted in Section 4 should be applied for the leading hexadecimal part (i.e., ::ffff:192.0.2.1 instead of 0:0:0:0:0:ffff:192.0.2.1).