Networks and addresses
IPv4 vs IPv6
Why there are two versions of the internet protocol, what IPv6 changes in practice, and why a server should have both.
IPv4 is the original internet protocol and still carries most traffic. Its 32-bit addresses are exhausted: the regional registries have none left to hand out, so new networks buy them second-hand or share them through CGNAT.
IPv6 fixes the shortage with 128-bit addresses and removes the need for NAT: every device can have a public address, every customer gets a whole prefix, and end-to-end connections work the way the internet was designed. Most mobile networks and a growing share of fixed lines use it by default; a site that lacks AAAA records is reached through translation by those users, or not at all.
The two do not talk to each other, so a server needs both to be reachable by everyone. On a dual-stack server the web server, mail server and DNS all listen on both, and the domain has A and AAAA records for each service.