In a dual-stack network, how does a DNS lookup decide between IPv6 and IPv4 — with a concrete example?

The resolver returns a AAAA record (IPv6) if one exists and an A record (IPv4) otherwise; e.g. hslu.ch had only an A record (147.88.201.68) so clients used IPv4, while dns.google has AAAA 2001:4860:4860::8888.

* DNS as the dual-stack switch: AAAA preferred, A as fallback. *

On a dual-stack host both protocols are available, and DNS is the switch:

• AAAA record → an IPv6 address (preferred when present)
• A record → an IPv4 address (the fallback)

Real example:

• nslookup hslu.ch → IPv4 147.88.201.68, but the AAAA query returned "no address" → the host can only reach it over IPv4
• nslookup dns.google → IPv6 2001:4860:4860::8888 and ::8844 exist → the host can use IPv6

This is exactly why migration is gradual: a dual-stack client happily uses IPv6 for sites that publish AAAA and silently falls back to IPv4 for those that don't.

Tip: 2001:4860:4860::8888 / ::8844 are Google Public DNS over IPv6 — the direct counterparts of 8.8.8.8 / 8.8.4.4.

Go deeper:

• IPv6 transition mechanism (Wikipedia) — dual-stack operation and how AAAA-vs-A drives protocol selection.