Механизм перехода в IPv6: 6to4

Internet transition mechanism

6to4 is an Internet transition mechanism for migrating from Internet Protocol version 4 (IPv4) to version 6 (IPv6) and a system that allows IPv6 packets to be transmitted over an IPv4 network (generally the IPv4 Internet) without the need to configure explicit tunnels. Special relay servers are also in place that allow 6to4 networks to communicate with native IPv6 networks. 6to4 is especially relevant during the initial phases of deployment to full, native IPv6 connectivity, since IPv6 is not required on nodes between the host and the destination. However, it is intended only as a transition mechanism and is not meant to be used permanently. 6to4 may be used by an individual host, or by a local IPv6 network. When used by a host, it must have a global IPv4 address connected, and the host is responsible for encapsulation of outgoing IPv6 packets and decapsulation of incoming 6to4 packets. If the host is configured to forward packets for other clients, often a local network, it is then a router. Most IPv6 networks use autoconfiguration, which requires the last 64 bits of the address for the host. The first 64 bits are the IPv6 prefix. The first 16 bits of the prefix are always :, the next 32 bits are the IPv4 address, and the last 16 bits of the prefix are available for addressing multiple IPv6 subnets behind the same 6to4 router. Since the IPv6 hosts using autoconfiguration already have determined the unique 64 bit host portion of their address, they must simply wait for a Router Advertisement indicating the first 64 bits of prefix to have a complete IPv6 address. A 6to4 router will know to send an encapsulated packet directly over IPv4 if the first 16 bits are , using the next 32 as the destination, or otherwise send the packet to a well known relay server, which has access to native IPv6. 6to4 does not facilitate interoperation between IPv4 only hosts and IPv6 only hosts (for that, see NAT64). 6to4 is simply a transparent mechanism used as a transport layer between IPv6 nodes. Due to the high levels of misconfigured hosts and poor performance observed, an advisory about how 6to4 should be deployed was published in August 2011. Due to unsolvable operational problems using the 6to4 anycast prefix, that part of the standard was deprecated in 2015.

For any 32 bit global IPv4 address that is assigned to a host, a 48 bit 6to4 IPv6 prefix can be constructed for use by that host (and if applicable the network behind it) by appending the IPv4 address to
For example, the global IPv4 address has the corresponding 6to4 prefix This gives a prefix length of 48 bits, which leaves room for a 16 bit subnet field and 64 bit host addresses within the subnets. Any IPv6 address that begins with the prefix (in other words, any address with the first two octets of 2002 hexadecimal) is known as a 6to4 address, as opposed to a native IPv6 address which does not systematically use transition technologies. Note that using a reserved IPv4 address, such as those provided by , is undefined, since these networks are disallowed from being routed on the public Internet. For example, using as the router's WAN address would be invalid since a return packet would not be able to determine the destination IPv4 address of the actual sender.

6to4 embeds an IPv6 packet in the payload portion of an IPv4 packet with protocol type 41. To send an IPv6 packet over an IPv4 network to a 6to4 destination address, an IPv4 header with protocol type 41 is prepended to the IPv6 packet. The IPv4 destination address for the prepended packet header is derived from the IPv6 destination address of the inner packet (which is in the format of a 6to4 address), by extracting the 32 bits immediately following the IPv6 destination address's prefix. The IPv4 source address in the prepended packet header is the IPv4 address of the host or router which is sending the packet over IPv4. The resulting IPv4 packet is then routed to its IPv4 destination address just like any other IPv4 packet.

To allow hosts and networks using 6to4 addresses to exchange traffic with hosts using "native" IPv6 addresses, "relay routers" have been established. A relay router connects to an IPv4 network and an IPv6 network. 6to4 packets arriving on an IPv4 interface will have their IPv6 payloads routed to the IPv6 network, while packets arriving on the IPv6 interface with a destination address prefix of will be encapsulated and forwarded over the IPv4 network. There is a difference between a "relay router" and a "border router" (also known as a "6to4 border router"). A 6to4 border router is an IPv6 router supporting a 6to4 pseudo interface. It is normally the border router between an IPv6 site and a wide area IPv4 network, where the IPv6 site uses co related to the IPv4 address used later on. On the other hand, a "relay router" is a 6to4 router configured to support transit routing between 6to4 addresses and pure native IPv6 addresses. To allow a 6to4 host to communicate with the native IPv6 Internet, it must have its IPv6 default gateway set to a 6to4 address which contains the IPv4 address of a 6to4 relay router. To avoid the need for users to set this up manually, the anycast address used to be allocated. It could be assigned to any number of 6to4 relay routers, so that IPv4 routing would take care of routing the encapsulated IPv6 packets to the one closest by for forwarding onto IPv6 Internet. Due to unsolvable operational problems, this use has been deprecated.