dual stack
termnetworkingISP & telecom
Running IPv4 and IPv6 side by side on the same hosts and links - the standard transition strategy while both internets coexist.
The pragmatic middle of the IPv6 transition, ahead of translation (NAT64) and tunneling approaches.
Dual stack runs IPv4 and IPv6 simultaneously on the same host, which is the transition mechanism that actually worked after the tunnelling and translation schemes proved awkward. A dual-stacked client prefers IPv6 where both are available and falls back without the user noticing.
The failure it introduces is a broken IPv6 path that is worse than no IPv6 at all: the client prefers a route that does not work and waits for a timeout before retrying. Happy Eyeballs is the fix, racing both address families and using whichever answers first, which turns a broken path into a slight delay. Dual stack also doubles the operational surface, since every firewall rule, monitoring check and ACL now exists twice and the second copy is the one nobody audits.