My website only has an IPv4 address. With IPv6 being the future, is it possible that some users may not be able to reach the website if it does not have an IPv6 address? Also, does having an IPv4/IPv6 address increase SEO performance?

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    "yes" <-- too short for an answer. – Criggie Oct 16 at 0:01
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    IPv6 is the present, IPv4 is legacy. – Martin Schröder Oct 17 at 20:16

Lack of IPv6 support on your site will hurt some of your users.

According to stats published by Google 20-25% of users currently have IPv6. A large fraction of those users will need to go through some kind of NAT to reach IPv4-only services, which will make connectivity less reliable.

The fraction of users without any IPv4 connectivity whatsoever is negligible. But for those users who have both IPv4 and IPv6 connectivity your site will be more reliable if you support both as well. Many clients now support RFC 6555 which will allow them to automatically fail over between IPv4 and IPv6.

Also keep in mind that if a network with both IPv4 and IPv6 support has a DHCP server outage the users on that network will still be able to reach mainstream sites, but if your site is IPv4-only they will not be able to reach your site. Thus they might conclude your site is down and not realize there is a problem on their network.

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    @MichaelHampton It depends on what you consider a mainstream site, which varies among users. StackExchange is the only reason I haven't tried running my home network IPv4-only without even DNS64. – kasperd Oct 15 at 12:55
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    "most" is an exageration, for the top 500 sites about 30% support IPv6. However the general point is correct, having working v6 but broken v4 is going to be very confusing to users. – Peter Green Oct 15 at 14:48
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    Some months ago I had some kind of semi-outage here which left IPv6 working, but IPv4 broken. It was quite interesting to observe which sites continued to work and which ones didn't. – Paŭlo Ebermann Oct 15 at 20:52
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    Although "hurt" may be too strong a word for this. From experience, I'd say almost all users don't notice when they're on an ISP's NAT in the first place. – tudor Oct 16 at 1:12
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    @tudor That's the problem. Most users don't know what NAT is or what it looks like. And of those who with enough technical knowledge to notice the NAT the majority have never seen a network without NAT and thinks this is how the internet was intended to work. But all of those users are still going to be affected by the problems caused by NAT. They will get annoyed at those problems, but they will have no clue what caused the problems. – kasperd Oct 16 at 7:16

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