What is dual-stack mode in Linux networking?

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Accepted Answer

Dual-stack means the host runs IPv4 and IPv6 in parallel on the same interfaces, so it can reach both IPv4-only and IPv6-only peers.
The internet is mid-migration: IPv4 was the dominant protocol as of the mid-2020s, but its ~4.3 billion addresses ran out, and IPv6 is the long-term replacement. You can't flip everything over at once, so the practical bridge is to speak both at the same time — that's dual-stack.
Why it's the default approach:

a dual-stack host talks to legacy IPv4-only services and newer IPv6-only ones
it allows a gradual, no-flag-day transition — networks add IPv6 without dropping IPv4
both stacks coexist on one interface, each with its own address(es) and routing

Red Hat Enterprise Linux ships in dual-stack mode out of the box, and most modern Linux distributions do the same — no special configuration is needed to have both.
Tip: when a host has both, applications generally try IPv6 first and fall back to IPv4 (the "Happy Eyeballs" approach), so connectivity is preserved either way.