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IPv6 (Internet Protocol version 6) is the Internet Protocol designed to succeed IPv4. It identifies network interfaces with 128-bit addresses and defines how packets are structured and forwarded across networks. The IETF specifies IPv6 in RFC 8200; its address architecture is described separately in RFC 4291.
What does IPv6 mean?
IPv6 stands for Internet Protocol version 6. It is a set of networking rules, not a device or service: equipment and software can implement IPv6 so they can send and receive packets using the protocol. The IETF describes it as a new version of IP designed as IPv4’s successor in RFC 8200, published in July 2017.
IPv6 was developed in the context of concern about the limited supply of IPv4 addresses. The NIST overview provides historical and technical context for that development.
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An IPv6 address is 128 bits long; an IPv4 address is 32 bits long. The larger width creates a much larger address space, but it does not mean every possible address is available or suitable for assigning to an ordinary host. The address format and architecture are specified in RFC 4291.
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IPv6 addresses identify network interfaces or sets of interfaces, rather than simply identifying a whole computer. RFC 4291 defines three basic address types:
- 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 for packet delivery.
IPv6 does not define broadcast as a fourth address type; multicast serves communication with a group of interfaces.
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How does an IPv6 packet work?
Every IPv6 packet has a base header. The header begins with a four-bit version field set to 6, and includes traffic class, flow label, payload length, next header, hop limit, and 128-bit source and destination addresses. RFC 8200 specifies this structure.
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The next-header field indicates what follows the base header: it may identify an extension header or an upper-layer protocol header. Extension headers carry optional internet-layer information. The hop limit is reduced as a packet passes through forwarding devices, bounding how far it can travel. For the full specification, see RFC 8200; its Hop-by-Hop Options processing provisions were updated by RFC 9673.
IPv6 and IPv4 at a glance
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address width | 32 bits | 128 bits |
| Protocol role | Internet Protocol version 4 | Internet Protocol version 6, designed as IPv4’s successor |
| Packet structure | Defined by the IPv4 specification | Base header with optional extension headers |
| Address types | Not detailed in the cited IPv6 address architecture | Unicast, anycast, and multicast, as defined in RFC 4291 |
The comparison describes protocol definitions; it does not establish that IPv6 automatically makes a connection faster or more secure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the definition does—and does not—tell you
IPv6 defines addressing and packet handling. Whether a particular device, network, or internet service supports it depends on that implementation; the definition alone does not establish current adoption rates or guarantee a performance or security outcome.
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