IPv6 (Internet Protocol version 6) is the version of the Internet Protocol designed to succeed IPv4. It uses 128-bit addresses to identify network interfaces and defines how IP packets are structured and forwarded. The Internet Engineering Task Force (IETF) specifies IPv6 in RFC 8200.
What does IPv6 mean?
IPv6 is a network-layer protocol: a set of rules that lets devices send packets across interconnected networks. It is a protocol specification, not a device or service. Network equipment and software can implement IPv6, just as they can implement other networking protocols.
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The IETF describes IPv6 as a new version of IP designed as IPv4’s successor. The specification, RFC 8200, was published in July 2017. IPv6’s addressing architecture is set out separately in RFC 4291, published in February 2006.
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An IPv6 address is 128 bits long; an IPv4 address is 32 bits long. That difference creates a vastly larger address space, but it does not mean every possible IPv6 address is available or used for an ordinary device. Addresses identify network interfaces, or sets of interfaces, rather than simply naming a whole computer. RFC 4291 defines three basic IPv6 address types:
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- Unicast: identifies one interface.
- Anycast: is assigned to a set of interfaces; routing delivers a packet to one member of that set.
- Multicast: identifies a set of interfaces to which packets can be delivered.
These are the address types in IPv6’s addressing architecture; IPv6 does not define broadcast as a fourth type.
What is in an IPv6 packet?
An IPv6 packet begins with a base header containing the information needed to handle and forward it. The header includes the version, traffic class, flow label, payload length, next header, hop limit, and source and destination addresses. The version field is four bits and has the value 6; each address field is 128 bits.
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- Next header indicates what follows the base header: an extension header or an upper-layer protocol header.
- Hop limit is reduced as the packet is forwarded, bounding how far it can travel.
- Extension headers carry optional internet-layer information when needed.
RFC 8200 specifies this base format and the extension-header framework. Its record notes that it was updated by RFC 9673 on Hop-by-Hop Options processing, so RFC 8200 should not be treated as the final word on every detail of that specific topic.
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IPv6 and IPv4 at a glance
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address width | 32 bits | 128 bits |
| Address types | Not detailed here | Unicast, anycast, and multicast, as defined by RFC 4291 |
| Packet header | Not detailed here | A base header with optional extension headers, specified by RFC 8200 |
The longer IPv6 address width is a defining difference, not a guarantee of faster or more secure connections. Those outcomes depend on how a network and its services are configured and used.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why was IPv6 developed?
IPv6 was developed as IPv4’s successor amid concern about IPv4 address depletion. The U.S. National Institute of Standards and Technology provides historical and technical background in its Internet Protocol Version 6 (IPv6) publication. That background does not establish a current IPv6 adoption rate.
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