IPv4 mapped address

IPv4 mapped addresses constitute a special class of IPv6 addresses. This address type has its first 80 bits set to zero, the next 16 set to one, while its last 32 bits represent an IPv4 address. For example, ::ffff:c000:280 is the IPv4 mapped address for the IPv4 address 192.0.2.128.

Notation

As a special exception to IPv6 addresses notation, IPv4 mapped addresses are commonly represented with their last 32 bits written in dot-decimal notation appended to the standard IPv6 notation of the leading bits, e.g., ::ffff:c000:280 could be written as ::ffff:192.0.2.128.

Usage

IPv4 mapped addresses are normally used in dual-stack Internet Protocol implementations to represent IPv4 addresses to IPv6 applications. It allows the transparent use of Transport Layer protocols (TCP or UDP) over IPv4 through the IPv6 networking API. It is therefore considered an IPv6 transition mechanism for Internet hosts.

An advantagous feature of this mechanism is that server applications only need to open a single listening socket to handle connections from clients using IPv6 or IPv4 protocols. IPv6 clients will be handled natively by default, and IPv4 clients appear as IPv6 clients with an appropriately mapped IPv6 address. It can also be used to establish IPv4 connections specifically with an IPv6 socket.

While the actual protocol on the network is IPv4, the connection will be presented as an IPv6 interface to the application.

Limitations

Because of the significant internal differences between IPv4 and IPv6 at all levels of the IP stack, some of the lower level functionality that may be exposed by the IPv6 stack might not work with IPv4 mapped addresses, if there is no direct translation to IPv4.

Some common IPv6 stacks do not support the IPv4 mapped address feature, either because the IPv6 and IPv4 stacks are separate implementations (Microsoft Windows prior to Vista/Longhorn: e.g. XP/2003), or because of security concerns (OpenBSD). On these operating systems, it is necessary to open a separate socket for each IP protocol that is to be supported. On some systems (e.g., NetBSD, FreeBSD) this feature is controlled by the socket option "IPV6_V6ONLY" as specified in RFC 3493.

The IETF Internet-Drafts mentioned below allay any fears regarding "security concerns" of mapped addresses, as long as the OS follows certain guidelines.

ee also

*IPv4-compatible address
* RFC 4942 - IPv6 Transition/Coexistence Security Considerations
* [http://tools.ietf.org/html/draft-itojun-v6ops-v4mapped-harmful draft-itojun-v6ops-v4mapped-harmful]
* [http://blogs.msdn.com/wndp/archive/2006/10/24/creating-ip-agnostic-applications-part-2-dual-mode-sockets.aspx Microsoft blog/tech note on mapped address use with dual mode sockets]