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Dual-Stack

Also known as: Dual-stack networking

A networking configuration where a device, server, or network runs both IPv4 and IPv6 simultaneously, allowing it to communicate with endpoints on either protocol.

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What is dual-stack?

Dual-stack is a network configuration where a host runs both IPv4 and IPv6 at the same time, with its own address in each protocol. Every modern operating system (Windows, macOS, Linux, iOS, Android) supports dual-stack by default. When the host needs to reach another server, it does a DNS lookup for both A (IPv4) and AAAA (IPv6) records and picks whichever protocol the destination supports.

Why dual-stack is the current default

The internet cannot migrate from IPv4 to IPv6 overnight. Billions of legacy devices only speak IPv4; billions of newer devices prefer IPv6. Dual-stack lets both worlds coexist:

  • IPv6-capable client → IPv6-capable server — use IPv6 directly, native and efficient
  • IPv6-capable client → IPv4-only server — fall back to IPv4
  • IPv4-only client → dual-stack server — use IPv4
  • IPv4-only client → IPv6-only server — needs a translation gateway (NAT64 / DNS64), rare in practice

Happy Eyeballs

Dual-stack hosts implement "Happy Eyeballs" (RFC 8305) — when connecting to a name that has both A and AAAA records, they start both TCP connections in parallel and use whichever completes first. This hides latency caused by broken IPv6 paths and gives users the fastest working route without manual tuning.

How dual-stack is deployed

  • Home ISPs — most major residential ISPs now offer dual-stack by default, with a public IPv4 address (often shared via CGNAT) and a routed IPv6 /56 or /64
  • Mobile carriers — Verizon, T-Mobile, and others operate IPv6-first networks with IPv4 fallback via NAT64
  • Cloud platforms — AWS, GCP, and Azure support dual-stack on VPCs, ALBs, and most services
  • CDN providers — Cloudflare, Fastly, Akamai all serve dual-stack by default

The long-term exit from dual-stack is IPv6-only deployment, where IPv4 exists only at translation gateways. Large corporate networks (Meta, LinkedIn, Microsoft's internal network) are already there. Residential and small-business networks are moving there more slowly. Our IP lookup tool reports both your IPv4 and IPv6 addresses when available, so you can see whether your connection is dual-stacked today.

Frequently Asked Questions

Dual-stack runs IPv4 and IPv6 in parallel — the host can reach destinations on either protocol natively. IPv6-only runs only IPv6 — IPv4 destinations are reached through a translation gateway (NAT64 + DNS64) that synthesizes IPv6 addresses from IPv4 ones. Dual-stack is the current mainstream because it just works with everything; IPv6-only is the long-term goal because it eliminates IPv4 entirely and is already running in large parts of mobile and enterprise networks.
Happy Eyeballs (RFC 8305) is the connection algorithm dual-stack hosts use when a hostname has both IPv4 and IPv6 addresses. Instead of trying IPv6 first and falling back to IPv4 on timeout (which produced multi-second hangs on broken IPv6 paths), Happy Eyeballs starts both connections in parallel with a small head start for IPv6, and uses whichever completes first. This hides broken IPv6 paths from users entirely. Every modern browser and OS implements it.
Eventually, yes — once IPv4 deployment shrinks enough that maintaining the IPv4 stack costs more than running translation gateways for the long tail. Large mobile networks and major tech companies (Meta, Microsoft internal, parts of Google) already run IPv6-only internally. Residential ISPs are moving more slowly because home router ecosystems are full of IPv4-only devices. Most projections put the long tail at 10-15 more years before dual-stack is rare.
Yes — at minimum one IPv4 and one IPv6, often more. A typical dual-stack home device has one private IPv4 (e.g. `192.168.1.50`) translated via NAT to share a single public IPv4, plus one or more globally routable IPv6 addresses assigned from the ISP-delegated /64 prefix. IPv6 also adds link-local addresses (`fe80::/10`) and often a temporary privacy address that rotates daily for outbound connections.
No, when paths are healthy. The Happy Eyeballs algorithm ensures the faster of the two protocols wins, with negligible overhead. The only case where dual-stack is measurably slower is when the IPv6 path is broken in a way that doesn't immediately fail (router that black-holes IPv6 traffic) — but Happy Eyeballs typically detects this within 50-300 ms and switches to IPv4. Pre-Happy-Eyeballs implementations did sometimes hang for several seconds on broken IPv6 paths; modern stacks do not.
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