Ask HN: Is not running a “dual stack” IPv4/IPv6 website costing me users? (yet)
If I have a website that can ONLY accept ipv4 what % of real world users cannot access it?
ipv4only 1h in a x.x.x.x
ipv6only 1h in aaaa x:x:x:x:x:x:x:x
If you're serving http(s) on standard ports, ipv4 only should be ok. Maybe not ideal, but ok. There's less and less high profile sites doing that, but at least for me, www.reddit.com and www.github.com are both returning only an A record right now; As of now, an internet connection that doesn't get you to github or reddit is clearly deficient, so most consumers have some access to ipv4.
ipv6 may be a little more important if you're doing stuff outside http. For users with ipv6, there's likely to be nat and other network manipulation and restriction on ipv6 vs ipv4. This can be pretty useful for say realtime communications or long mostly idle connections, where CGNAT can cause trouble.
These are two different things.
If your site is slow, it will cost you users. v6 has measurably better performance for websites, so not having it will cost you users. Those users will still be able to access your site though, they'll just choose not to because it's too slow for them.
The practical reality is that almost all client devices that are used for browsing the web will have some mechanism to reach legacy v4-only websites (but the mechanism might itself be a cause of poor performance...).
On the server side, v6-only servers (VPSs/containers/etc) are getting somewhat more common though, so if you're hosting something that's expected to be downloaded directly onto a server then please figure out v6 somehow.
(Not 1.4x faster overall, as you might think from the headline...)