IPv6

IPv6 solves the Internet's address-exhaustion problem by using 128-bit addresses, producing a vastly larger space than IPv4's 32-bit addresses.¹ The address is written as eight hexadecimal groups, with rules for suppressing consecutive zero groups and representing embedded IPv4 values. Its size permits large allocations to organizations and supports address aggregation, reducing pressure on routing tables.

The protocol's base header is fixed at 40 bytes and removes fields that were rarely useful or could be handled more efficiently elsewhere. Optional information is carried in extension headers, while routers generally forward packets without fragmenting them; fragmentation is performed by the sending host when required.¹ IPv6 does not itself guarantee encryption, reliability, or application behavior: those functions remain the responsibility of protocols such as TCP, TLS, or applications.

IPv6 defines several address scopes and delivery patterns rather than a single universal address type. Unicast identifies an interface, multicast delivers to members of a group, and anycast sends traffic to one member of a set, normally the topologically nearest one.² Global unicast addresses are routable on the public Internet, while link-local addresses begin with fe80::/10 and are valid only on the local link.

Every IPv6-enabled interface is expected to have a link-local address, even when it also has a globally routable one.² The address architecture also reserves special ranges for unique-local addressing, documentation, loopback, unspecified addresses, and transition mechanisms. There is no broadcast address; multicast groups provide the corresponding local delivery functions.

IPv6 replaces IPv4 Address Resolution Protocol with the Neighbor Discovery Protocol, which uses ICMPv6 messages to discover routers, resolve neighboring link-layer addresses, detect duplicate addresses, and learn network parameters.⁴ Hosts can configure addresses through Stateless Address Autoconfiguration, DHCPv6, or a combination of the two. Router advertisements can supply prefixes and default-router information without requiring a central address server.

Deployment commonly uses dual-stack operation, in which devices run IPv4 and IPv6 simultaneously. Tunneling, translation, and proxy techniques support networks where one protocol is absent, but they add operational complexity. IPv6's larger address space reduces the need for network-address translation, although NAT remains common during the long coexistence of both protocols. ICMPv6 must be filtered carefully: blocking it indiscriminately can break essential discovery and path-operation functions.

Some of IPv6's most consequential features are subtle rather than visible in an address. Extension headers permit capabilities such as routing options, fragmentation, and destination-specific processing, but their ordering and handling can affect firewalls and high-speed routers.¹ The protocol also includes a flow-label field intended to help identify packets belonging to a flow, although practical use varies among networks.

Privacy extensions can create temporary interface addresses that change over time, limiting long-term tracking based on a stable host identifier.⁵ Conversely, stable addresses may be useful for servers and network management. IPv6 is also used in constrained environments through standards such as 6LoWPAN, which compresses headers and adapts packets for low-power wireless links. These edge cases show that deployment is not merely an address-renumbering exercise but a change in operations, security policy, and device behavior.

128-bit address

An IPv6 address containing 128 binary bits, conventionally written as eight hexadecimal groups.

Link-local address

An address used for communication on a directly connected link; IPv6 link-local addresses use the fe80::/10 range.

Neighbor Discovery Protocol

An ICMPv6-based protocol for router discovery, address resolution, parameter discovery, and neighbor reachability.

SLAAC

Stateless Address Autoconfiguration, through which hosts form addresses from advertised network prefixes and interface information.

Anycast

A delivery method in which a packet addressed to one address is routed to one member of a configured group.