Dual Stack (IPv4 and IPv6)

Dual stack is a transition approach in which hosts, routers and services run both IPv4 और IPv6 at the same time. Each interface holds addresses of both families, and applications can reach destinations over either one.

Why it matters

IPv6 adoption is uneven, so most networks cannot switch off IPv4 yet. Dual stack lets a site serve both kinds of visitors without translation. When a name has both A and AAAA records, clients typically use an algorithm called Happy Eyeballs: they try IPv6 first and quickly fall back to IPv4 if it does not connect within a few hundred milliseconds, hiding most broken paths from users.

It also doubles the work for administrators: two address plans, two sets of firewall rules and two sets of monitoring.

Example

$ dig +short example.com A
203.0.113.10
$ dig +short example.com AAAA
2001:db8::10

Both records exist, so the service is reachable on both stacks. A common failure is a good AAAA record whose IPv6 firewall rules were never opened, so some users see slow or failed connections.

Common mistakes

Publishing an AAAA record without testing that the server answers on IPv6.
Securing IPv4 carefully and leaving IPv6 wide open.
Assuming IPv4 NAT rules also protect IPv6 hosts.

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