What are the parts of an IPv4 address

Internet Protocol version 4 is described in IETF publication RFC 791 (September 1981), replacing an earlier definition of January 1980 (RFC 760). In March 1982, the US Department of Defense decided on the Internet Protocol Suite (TCP/IP) as the standard for all military computer networking.[5]

The Internet Protocol is the protocol that defines and enables internetworking at the internet layer of the Internet Protocol Suite. In essence it forms the Internet. It uses a logical addressing system and performs routing, which is the forwarding of packets from a source host to the next router that is one hop closer to the intended destination host on another network.

IPv4 is a connectionless protocol, and operates on a best-effort delivery model, in that it does not guarantee delivery, nor does it assure proper sequencing or avoidance of duplicate delivery. These aspects, including data integrity, are addressed by an upper layer transport protocol, such as the Transmission Control Protocol (TCP).

IPv4 uses 32-bit addresses which limits the address space to 4294967296 (232) addresses.

IPv4 reserves special address blocks for private networks (~18 million addresses) and multicast addresses (~270 million addresses).

Address representations

IPv4 addresses may be represented in any notation expressing a 32-bit integer value. They are most often written in dot-decimal notation, which consists of four octets of the address expressed individually in decimal numbers and separated by periods.

For example, the quad-dotted IP address 192.0.2.235 represents the 32-bit decimal number 3221226219, which in hexadecimal format is 0xC00002EB.

CIDR notation combines the address with its routing prefix in a compact format, in which the address is followed by a slash character (/) and the count of leading consecutive 1 bits in the routing prefix (subnet mask).

Other address representations were in common use when classful networking was practiced. For example, the loopback address 127.0.0.1 is commonly written as 127.1, given that it belongs to a class-A network with eight bits for the network mask and 24 bits for the host number. When fewer than four numbers are specified in the address in dotted notation, the last value is treated as an integer of as many bytes as are required to fill out the address to four octets. Thus, the address 127.65530 is equivalent to 127.0.255.250.

Allocation

In the original design of IPv4, an IP address was divided into two parts: the network identifier was the most significant octet of the address, and the host identifier was the rest of the address. The latter was also called the rest field. This structure permitted a maximum of 256 network identifiers, which was quickly found to be inadequate.

To overcome this limit, the most-significant address octet was redefined in 1981 to create network classes, in a system which later became known as classful networking. The revised system defined five classes. Classes A, B, and C had different bit lengths for network identification. The rest of the address was used as previously to identify a host within a network. Because of the different sizes of fields in different classes, each network class had a different capacity for addressing hosts. In addition to the three classes for addressing hosts, Class D was defined for multicast addressing and Class E was reserved for future applications.

Dividing existing classful networks into subnets began in 1985 with the publication of RFC 950. This division was made more flexible with the introduction of variable-length subnet masks (VLSM) in RFC 1109 in 1987. In 1993, based on this work, RFC 1517 introduced Classless Inter-Domain Routing (CIDR),[6] which expressed the number of bits (from the most significant) as, for instance, /24, and the class-based scheme was dubbed classful, by contrast. CIDR was designed to permit repartitioning of any address space so that smaller or larger blocks of addresses could be allocated to users. The hierarchical structure created by CIDR is managed by the Internet Assigned Numbers Authority (IANA) and the regional Internet registries (RIRs). Each RIR maintains a publicly searchable WHOIS database that provides information about IP address assignments.

Special-use addresses

The Internet Engineering Task Force (IETF) and IANA have restricted from general use various reserved IP addresses for special purposes.[7] Notably these addresses are used for multicast traffic and to provide addressing space for unrestricted uses on private networks.

Private networks

Of the approximately four billion addresses defined in IPv4, about 18 million addresses in three ranges are reserved for use in private networks. Packets addresses in these ranges are not routable in the public Internet; they are ignored by all public routers. Therefore, private hosts cannot directly communicate with public networks, but require network address translation at a routing gateway for this purpose.

Since two private networks, e.g., two branch offices, cannot directly interoperate via the public Internet, the two networks must be bridged across the Internet via a virtual private network (VPN) or an IP tunnel, which encapsulates packets, including their headers containing the private addresses, in a protocol layer during transmission across the public network. Additionally, encapsulated packets may be encrypted for transmission across public networks to secure the data.

Link-local addressing

RFC 3927 defines the special address block 169.254.0.0/16 for link-local addressing. These addresses are only valid on the link (such as a local network segment or point-to-point connection) directly connected to a host that uses them. These addresses are not routable. Like private addresses, these addresses cannot be the source or destination of packets traversing the internet. These addresses are primarily used for address autoconfiguration (Zeroconf) when a host cannot obtain an IP address from a DHCP server or other internal configuration methods.

When the address block was reserved, no standards existed for address autoconfiguration. Microsoft created an implementation called Automatic Private IP Addressing (APIPA), which was deployed on millions of machines and became a de facto standard. Many years later, in May 2005, the IETF defined a formal standard in RFC 3927, entitled Dynamic Configuration of IPv4 Link-Local Addresses.

Loopback

Main article: Localhost

The class A network 127.0.0.0 (classless network 127.0.0.0/8) is reserved for loopback. IP packets whose source addresses belong to this network should never appear outside a host. Packets received on a non-loopback interface with a loopback source or destination address must be dropped.

First and last subnet addresses

See also: IPv4 subnetting reference

The first address in a subnet is used to identify the subnet itself. In this address all host bits are 0. To avoid ambiguity in representation, this address is reserved.[20] The last address has all host bits set to 1. It is used as a local broadcast address for sending messages to all devices on the subnet simultaneously. For networks of size/24 or larger, the broadcast address always ends in 255.

For example, in the subnet 192.168.5.0/24 (subnet mask 255.255.255.0) the identifier 192.168.5.0 is used to refer to the entire subnet. The broadcast address of the network is 192.168.5.255.

However, this does not mean that every address ending in 0 or 255 cannot be used as a host address. For example, in the/16 subnet 192.168.0.0/255.255.0.0, which is equivalent to the address range 192.168.0.0–192.168.255.255, the broadcast address is 192.168.255.255. One can use the following addresses for hosts, even though they end with 255: 192.168.1.255, 192.168.2.255, etc. Also, 192.168.0.0 is the network identifier and must not be assigned to an interface.[21] The addresses 192.168.1.0, 192.168.2.0, etc., may be assigned, despite ending with 0.

In the past, conflict between network addresses and broadcast addresses arose because some software used non-standard broadcast addresses with zeros instead of ones.[22]

In networks smaller than /24, broadcast addresses do not necessarily end with 255. For example, a CIDR subnet 203.0.113.16/28 has the broadcast address 203.0.113.31.

As a special case, a /31 network has capacity for just two hosts. These networks are typically used for point-to-point connections. There is no network identifier or broadcast address for these networks.[23]

Address resolution

Main article: Domain Name System

Hosts on the Internet are usually known by names, e.g., www.example.com, not primarily by their IP address, which is used for routing and network interface identification. The use of domain names requires translating, called resolving, them to addresses and vice versa. This is analogous to looking up a phone number in a phone book using the recipient's name.

The translation between addresses and domain names is performed by the Domain Name System (DNS), a hierarchical, distributed naming system that allows for the subdelegation of namespaces to other DNS servers.

Unnumbered interface

A unnumbered point-to-point (PtP) link, also called a transit link, is a link that doesn't have an IP network or subnet number associated with it, but still has an IP address. First introduced in 1993,[24][25][26][27][28] Phil Karn from Qualcomm is credited as the original designer.

The purpose of a transit link is to route datagrams. They are used to free IP addresses from a scarce IP address space or to reduce the management of assigning IP and configuration of interfaces. Previously, every link needed to dedicated/30 or/31 subnet using 2-4 IP addresses per point-to-point link. When a link is unnumbered, a router-id is used, a single IP address borrowed from a defined (normally a loopback) interface. The same router-id can be used on multiple interfaces.

One of the disadvantages of unnumbered interfaces is that it is harder to do remote testing and management.

Main article: IPv4 address exhaustion

In the 1980s, it became apparent that the pool of available IPv4 addresses was depleting at a rate that was not initially anticipated in the original design of the network.[29] The main market forces that accelerated address depletion included the rapidly growing number of Internet users, who increasingly used mobile computing devices, such as laptop computers, personal digital assistants (PDAs), and smart phones with IP data services. In addition, high-speed Internet access was based on always-on devices. The threat of exhaustion motivated the introduction of a number of remedial technologies, such as:

• Classless Inter-Domain Routing (CIDR), for smaller ISP allocations
• Unnumbered interface, removed the need on transit links.
• network address translation, removed the need for end-to-end principle.

By the mid-1990s, network address translation (NAT) was used pervasively in network access provider systems, along with strict usage-based allocation policies at the regional and local Internet registries.

The primary address pool of the Internet, maintained by IANA, was exhausted on 3 February 2011, when the last five blocks were allocated to the five RIRs.[30][31] APNIC was the first RIR to exhaust its regional pool on 15 April 2011, except for a small amount of address space reserved for the transition technologies to IPv6, which is to be allocated under a restricted policy.[32]

The long-term solution to address exhaustion was the 1998 specification of a new version of the Internet Protocol, IPv6.[33] It provides a vastly increased address space, but also allows improved route aggregation across the Internet, and offers large subnetwork allocations of a minimum of 264 host addresses to end users. However, IPv4 is not directly interoperable with IPv6, so that IPv4-only hosts cannot directly communicate with IPv6-only hosts. With the phase-out of the 6bone experimental network starting in 2004, permanent formal deployment of IPv6 commenced in 2006.[34] Completion of IPv6 deployment is expected to take considerable time,[35] so that intermediate transition technologies are necessary to permit hosts to participate in the Internet using both versions of the protocol.

An IP packet consists of a header section and a data section. An IP packet has no data checksum or any other footer after the data section. Typically the link layer encapsulates IP packets in frames with a CRC footer that detects most errors, many transport-layer protocols carried by IP also have their own error checking.[36]

Header

The IPv4 packet header consists of 14 fields, of which 13 are required. The 14th field is optional and aptly named: options. The fields in the header are packed with the most significant byte first (big endian), and for the diagram and discussion, the most significant bits are considered to come first (MSB 0 bit numbering). The most significant bit is numbered 0, so the version field is actually found in the four most significant bits of the first byte, for example.

• bit 0: Reserved; must be zero.[b]
• bit 1: Don't Fragment (DF)
• bit 2: More Fragments (MF)

Data

The packet payload is not included in the checksum. Its contents are interpreted based on the value of the Protocol header field.

List of IP protocol numbers contains a complete list of payload protocol types. Some of the common payload protocols include:

Main article: IP fragmentation

The Internet Protocol enables traffic between networks. The design accommodates networks of diverse physical nature; it is independent of the underlying transmission technology used in the link layer. Networks with different hardware usually vary not only in transmission speed, but also in the maximum transmission unit (MTU). When one network wants to transmit datagrams to a network with a smaller MTU, it may fragment its datagrams. In IPv4, this function was placed at the Internet Layer and is performed in IPv4 routers limiting exposure to these issues by hosts.

In contrast, IPv6, the next generation of the Internet Protocol, does not allow routers to perform fragmentation; hosts must perform Path MTU Discovery before sending datagrams.

Fragmentation

When a router receives a packet, it examines the destination address and determines the outgoing interface to use and that interface's MTU. If the packet size is bigger than the MTU, and the Do not Fragment (DF) bit in the packet's header is set to 0, then the router may fragment the packet.

The router divides the packet into fragments. The maximum size of each fragment is the outgoing MTU minus the IP header size (20 bytes minimum; 60 bytes maximum). The router puts each fragment into its own packet, each fragment packet having the following changes:

• The total length field is the fragment size.
• The more fragments (MF) flag is set for all fragments except the last one, which is set to 0.
• The fragment offset field is set, based on the offset of the fragment in the original data payload. This is measured in units of 8-byte blocks.
• The header checksum field is recomputed.

For example, for an MTU of 1,500 bytes and a header size of 20 bytes, the fragment offsets would be multiples of 1,500 − 20 8 = 185 {\displaystyle {\frac {1{,}500-20}{8}}=185}   (0, 185, 370, 555, 740, etc.).

It is possible that a packet is fragmented at one router, and that the fragments are further fragmented at another router. For example, a packet of 4,520 bytes, including a 20 bytes IP header is fragmented to two packets on a link with an MTU of 2,500 bytes:

The total data size is preserved: 2,480 bytes + 2,020 bytes = 4,500 bytes. The offsets are 0 {\displaystyle 0}   and 0 + 2,480 8 = 310 {\displaystyle {\frac {0+2{,}480}{8}}=310}  .

When forwarded to a link with an MTU of 1,500 bytes, each fragment is fragmented into two fragments:

Again, the data size is preserved: 1,480 + 1,000 = 2,480, and 1,480 + 540 = 2,020.

Also in this case, the More Fragments bit remains 1 for all the fragments that came with 1 in them and for the last fragment that arrives, it works as usual, that is the MF bit is set to 0 only in the last one. And of course, the Identification field continues to have the same value in all re-fragmented fragments. This way, even if fragments are re-fragmented, the receiver knows they have initially all started from the same packet.

The last offset and last data size are used to calculate the total data size: 495 × 8 + 540 = 3,960 + 540 = 4,500 {\displaystyle 495\times 8+540=3{,}960+540=4{,}500}  .

Reassembly

A receiver knows that a packet is a fragment, if at least one of the following conditions is true:

• The flag more fragments is set, which is true for all fragments except the last.
• The field fragment offset is nonzero, which is true for all fragments except the first.

The receiver identifies matching fragments using the source and destination addresses, the protocol ID, and the identification field. The receiver reassembles the data from fragments with the same ID using both the fragment offset and the more fragments flag. When the receiver receives the last fragment, which has the more fragments flag set to 0, it can calculate the size of the original data payload, by multiplying the last fragment's offset by eight and adding the last fragment's data size. In the given example, this calculation was 495 × 8 + 540 = 4,500 {\displaystyle 495\times 8+540=4{,}500}   bytes. When the receiver has all fragments, they can be reassembled in the correct sequence according to the offsets to form the original datagram.

IP addresses are not tied in any permanent manner to networking hardware and, indeed, in modern operating systems, a network interface can have multiple IP addresses. In order to properly deliver an IP packet to the destination host on a link, hosts and routers need additional mechanisms to make an association between the hardware address[c] of network interfaces and IP addresses. The Address Resolution Protocol (ARP) performs this IP-address-to-hardware-address translation for IPv4. In addition, the reverse correlation is often necessary. For example, unless an address is preconfigured by an administrator, when an IP host is booted or connected to a network it needs to determine its IP address. Protocols for such reverse correlations include Dynamic Host Configuration Protocol (DHCP), Bootstrap Protocol (BOOTP) and, infrequently, reverse ARP.

• History of the Internet
• List of assigned /8 IPv4 address blocks

1. ^ Updated by RFC 3168 and RFC 3260
2. ^ As an April Fools' joke, proposed for use in RFC 3514 as the "Evil bit"
3. ^ For IEEE 802 networking technologies, including Ethernet, the hardware address is a MAC address.

1. ^ "BGP Analysis Reports". Retrieved 2013-01-09.
2. ^ "IPv6 – Google". www.google.com. Retrieved 2022-01-28.
3. ^ "IANA IPv4 Special-Purpose Address Registry". www.iana.org. Retrieved 2022-01-28.
4. ^ "RFC 5735 - Special Use IPv4 Addresses". datatracker.ietf.org. Retrieved 2022-01-28.
5. ^ "A Brief History of IPv4". IPv4 Market Group. Retrieved 2020-08-19.
6. ^ "Understanding IP Addressing: Everything You Ever Wanted To Know" (PDF). 3Com. Archived from the original (PDF) on June 16, 2001.
7. ^ Cotton, M.; Vegoda, L. (January 2010). Special Use IPv4 Addresses. doi:10.17487/RFC5735. RFC 5735.
8. ^ a b c d M. Cotton; L. Vegoda; R. Bonica; B. Haberman (April 2013). Special-Purpose IP Address Registries. Internet Engineering Task Force. doi:10.17487/RFC6890. BCP 153. RFC 6890. Updated by RFC 8190.
9. ^ a b c d Y. Rekhter; B. Moskowitz; D. Karrenberg; G. J. de Groot; E. Lear (February 1996). Address Allocation for Private Internets. Network Working Group. doi:10.17487/RFC1918. BCP 5. RFC 1918. Updated by RFC 6761.
10. ^ J. Weil; V. Kuarsingh; C. Donley; C. Liljenstolpe; M. Azinger (April 2012). IANA-Reserved IPv4 Prefix for Shared Address Space. Internet Engineering Task Force (IETF). doi:10.17487/RFC6598. ISSN 2070-1721. BCP 153. RFC 6598.
11. ^ S. Cheshire; B. Aboba; E. Guttman (May 2005). Dynamic Configuration of IPv4 Link-Local Addresses. Network Working Group. doi:10.17487/RFC3927. RFC 3927.
12. ^ a b c J. Arkko; M. Cotton; L. Vegoda (January 2010). IPv4 Address Blocks Reserved for Documentation. Internet Engineering Task Force. doi:10.17487/RFC5737. ISSN 2070-1721. RFC 5737.
13. ^ O. Troan (May 2015). B. Carpenter (ed.). Deprecating the Anycast Prefix for 6to4 Relay Routers. Internet Engineering Task Force. doi:10.17487/RFC7526. BCP 196. RFC 7526.
14. ^ C. Huitema (June 2001). An Anycast Prefix for 6to4 Relay Routers. Network Working Group. doi:10.17487/RFC3068. RFC 3068. Obsoleted by RFC 7526.
15. ^ S. Bradner; J. McQuaid (March 1999). Benchmarking Methodology for Network Interconnect Devices. Network Working Group. doi:10.17487/RFC2544. RFC 2544. Updated by: RFC 6201 and RFC 6815.
16. ^ a b M. Cotton; L. Vegoda; D. Meyer (March 2010). IANA Guidelines for IPv4 Multicast Address Assignments. Internet Engineering Task Force. doi:10.17487/RFC5771. BCP 51. RFC 5771.
17. ^ S. Venaas; R. Parekh; G. Van de Velde; T. Chown; M. Eubanks (August 2012). Multicast Addresses for Documentation. Internet Engineering Task Force. doi:10.17487/RFC6676. RFC 6676.
18. ^ J. Reynolds, ed. (January 2002). Assigned Numbers: RFC 1700 is Replaced by an On-line Database. Network Working Group. doi:10.17487/RFC3232. RFC 3232. Obsoletes RFC 1700.
19. ^ Jeffrey Mogul (October 1984). Broadcasting Internet Datagrams. Network Working Group. doi:10.17487/RFC0919. RFC 919.
20. ^ "RFC 923". IETF. June 1984. Retrieved 15 November 2019. Special Addresses: In certain contexts, it is useful to have fixed addresses with functional significance rather than as identifiers of specific hosts. When such usage is called for, the address zero is to be interpreted as meaning "this", as in "this network".
21. ^ Robert Braden (October 1989). "Requirements for Internet Hosts – Communication Layers". IETF. p. 31. RFC 1122.
22. ^ Robert Braden (October 1989). "Requirements for Internet Hosts – Communication Layers". IETF. p. 66. RFC 1122.
23. ^ RFC 3021
24. ^ Almquist, Philip; Kastenholz, Frank (November 1994). "Towards Requirements for IP Routers". Internet Engineering Task Force.
25. ^ RFC 1916
26. ^ RFC 1716
27. ^ RFC 1812
28. ^ "Understanding and Configuring the ip unnumbered Command". Cisco. Retrieved 2021-11-25.
29. ^ "World 'running out of Internet addresses'". Archived from the original on 2011-01-25. Retrieved 2011-01-23.
30. ^ Smith, Lucie; Lipner, Ian (3 February 2011). "Free Pool of IPv4 Address Space Depleted". Number Resource Organization. Retrieved 3 February 2011.
31. ^ ICANN,nanog mailing list. "Five /8s allocated to RIRs – no unallocated IPv4 unicast /8s remain".
32. ^ Asia-Pacific Network Information Centre (15 April 2011). "APNIC IPv4 Address Pool Reaches Final /8". Archived from the original on 7 August 2011. Retrieved 15 April 2011.
33. ^ Hinden, Bob; Deering, Steve E. (December 1998). "Internet Protocol, Version 6 (IPv6) Specification". tools.ietf.org. Retrieved 2019-12-13.
34. ^ Fink, R.; HInden, R. (March 2004). 6bone (IPv6 Testing Address Allocation) Phaseout. doi:10.17487/RFC3701. RFC 3701.
35. ^ 2016 IEEE International Conference on Emerging Technologies and Innovative Business Practices for the Transformation of Societies (EmergiTech). Piscataway, NJ: University of Technology, Mauritius, Institute of Electrical and Electronics Engineers. August 2016. ISBN 9781509007066. OCLC 972636788.
36. ^ Partridge, C.; Kastenholz, F. (December 1994). "6.2 IP Header Checksum". Technical Criteria for Choosing IP The Next Generation (IPng). p. 26. sec. 6.2. doi:10.17487/RFC1726. RFC 1726.
37. ^ Postel, J. Internet Protocol. doi:10.17487/RFC0791. RFC 791.
38. ^ Savage, Stefan (2000). "Practical network support for IP traceback". ACM SIGCOMM Computer Communication Review. 30 (4): 295–306. doi:10.1145/347057.347560. Retrieved 2010-09-06.
39. ^ "Cisco unofficial FAQ". Retrieved 2012-05-10.
40. ^ "Internet Protocol Version 4 (IPv4) Parameters".

IPv4 at Wikipedia's sister projects

•  Definitions from Wiktionary
•  Media from Commons
•  Resources from Wikiversity

• Internet Assigned Numbers Authority (IANA)
• IP, Internet Protocol — IP Header Breakdown, including specific options
• IP Mobility Support for IPv4. RFC 3344.
• Official current state of IPv4/8 allocations, as maintained by IANA

6to4 performs three functions:

• Assigns a block of IPv6 address space to any host or network that has a global IPv4 address.
• Encapsulates IPv6 packets inside IPv4 packets for transmission over an IPv4 network using 6in4.
• Routes traffic between 6to4 and "native" IPv6 networks.

Address block allocation

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 2002::/16.

For example, the global IPv4 address 192.0.2.4 has the corresponding 6to4 prefix 2002:c000:0204::/48. 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 2002::/16 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 RFC 1918, is undefined, since these networks are disallowed from being routed on the public Internet. For example, using 192.168.1.1 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.

Encapsulation and transmission

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 2002::/16 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.

Routing between 6to4 and native IPv6

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 2002::/16 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 2002::/16 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 192.88.99.1 used to be allocated.[4] 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.[2][3]

Packets from the IPv6 Internet to 6to4 systems must be sent to a 6to4 relay router by normal IPv6 routing methods. The specification states that such relay routers must only advertise 2002::/16 and not subdivisions of it to prevent IPv4 routes polluting the routing tables of IPv6 routers. From here they can then be sent over the IPv4 Internet to the destination.

For a 6to4 host to have fast and reliable connectivity with a host natively using the IPv6 Internet, both the 6to4 host and the native IPv6 host must have a route to a fast, reliable and correctly configured relay server. The 6to4 host's ISP can ensure that outgoing packets go to such a relay, but they have no control over the relay used for the responses from the native IPv6 host. A variant called IPv6 rapid deployment ("6rd") uses the same basic principles as 6to4 but uses a relay operated by the 6rd user's ISP for traffic in both directions. To achieve this an address block allocated by the user's ISP is used instead of 2002::/16.

When a site using 6to4 has a fixed global IPv4 address, its 6to4 IPv6 prefix is also fixed. It is then possible to request reverse DNS delegation for an individual 6to4 48-bits prefix inside the 2.0.0.2.ip6.arpa DNS zone from the Number Resource Organization at 6to4.nro.net. The process is entirely automatic.

According to RFC 3964, 6to4 routers and relays should ensure that:

• either or both the source and destination addresses of any encapsulated packet is within the 6to4 IPv6 prefix 2002::/16,
• if the source IPv6 address is a 6to4 IPv6 address, its corresponding 6to4 router IPv4 address matches the IPv4 source address in the IPv4 encapsulation header,
• similarly, if the destination IPv6 address is a 6to4 IPv6 address, its corresponding 6to4 router IPv4 address matches the IPv4 destination address in the IPv4 encapsulation header,
• any embedded 6to4 router IPv4 address is global unicast.

6to4 has been default disabled since the Anniversary Update. It is no longer being actively supported and might be removed in a future release.[5]

• IPv6 rapid deployment
• Teredo tunneling

• B. Carpenter & K. Moore. Connection of IPv6 Domains via IPv4 Clouds. RFC 3056, February 2001.
• R. Gilligan & E. Nordmark. Transition Mechanisms for IPv6 Hosts and Routers. RFC 2893, August 2000.
• P. Savola & C. Patel. Security Considerations for 6to4. RFC 3964, December 2004.
• B. Carpenter. Advisory Guidelines for 6to4 Deployment. RFC 6343, August 2011.
• O. Troan, B. Carpenter, Ed. Deprecating the Anycast Prefix for 6to4 Relay Routers. RFC 7526, May 2015.

1. ^ RFC 3056 - Connection of IPv6 Domains via IPv4 Clouds
2. ^ a b RFC 6343 - Advisory Guidelines for 6to4 Deployment
3. ^ a b RFC 7526 - Deprecating the Anycast Prefix for 6to4 Relay Routers
4. ^ C. Huitema (June 2001). An Anycast Prefix for 6to4 Relay Routers. Network Working Group. doi:10.17487/RFC3068. RFC 3068.
5. ^ greg-lindsay. "Windows 10 features we're no longer developing". docs.microsoft.com. Retrieved 2019-12-02.

• "Routing IPv6 over IPv4" article by Cisco Archived 2015-12-03 at the Wayback Machine
• IPv6 6to4 tunnel configuration example