IPv4 vs IPv6, and Why Some Sites Still Are Not Reachable Over It

IPv4 addresses are 32 bits long, which allows for a little over four billion unique addresses. That sounded effectively unlimited when the format was designed in the early 1980s, and turned out to be nowhere near enough once billions of phones, laptops and servers all needed one. IPv6 addresses are 128 bits long instead, which allows for more unique addresses than there are grains of sand on every beach on the planet several times over, comfortably solving the exhaustion problem for the foreseeable future.

A domain announces its IPv4 address through an A record and its IPv6 address, if it has one, through a separate AAAA record. The two exist independently of each other. A domain can have only an A record, only an AAAA record, or both, and a browser or other client will generally prefer IPv6 automatically when both are present and its own network supports it.

IPv6 has technically existed since the late 1990s, yet a meaningful share of websites, including some large ones, still have no AAAA record at all. The main reason is simply that IPv4 has not actually run out in any way that forces the issue for an individual site: providers ration remaining IPv4 addresses, share them across customers through techniques like carrier grade NAT, and the existing internet keeps functioning well enough that adding IPv6 support often stays low on the priority list indefinitely.

Enabling it also is not always a simple toggle. It can require support from the hosting provider, correct dual stack configuration on the server itself, and updates to firewall rules that were only ever written with IPv4 in mind, all for a change that, done correctly, most visitors will not consciously notice either way.

Increasingly, yes. A large and growing share of mobile networks and newer residential connections are IPv6 only or IPv6 preferred at the network level, relying on translation layers to reach IPv4 only destinations. A site with no AAAA record is not unreachable for these visitors, but it is taking a slower or less direct path to reach them than one that supports IPv6 natively. For a high traffic or latency sensitive site, that gap is worth knowing about even if it rarely causes an outright failure.

The IPv6 Test tool checks whether a domain has a public AAAA record, which is what determines whether it is reachable over IPv6 at all. It reports on the domain's own DNS configuration rather than testing a live connection, since real world reachability also depends on the network of whoever is actually connecting, not just the target's own setup.

dig example.com AAAA +short