Depends. Anecdotal evidence: Most "normal user" software handles it fine. (Chrome and Apple Mail on my Mac, Android apps, etc.) With terminal apps I use as a d

Depends. Anecdotal evidence: Most "normal user" software handles it fine. (Chrome and Apple Mail on my Mac, Android apps, etc.) With terminal apps I use as a developer, it's more of a crapshoot. curl and SSH work fine. I've seen others that don't. Notably, Node.js's socket libraries can't fall back to IPv4 at all, even after the v6 connection times out. Node's DNS library actually overrides the default system behavior to prefer v4 over v6, probably for this reason. (This can be changed globally within a particular program, or on a per-lookup basis, but preferring v6 will allegedly become the default in an upcoming release.) I've unfortunately had much experience with this due to major bugs in Comcast's modems.