The addressing system alongside IPv4.
WHAT IT GIVES YOU
An address space large enough that scarcity is not a concern, and direct connectivity for IPv6 clients.
CONFIGURING IT
If your server is allocated IPv6, configure the interface and add an AAAA record for your domain pointing at it.
Your web server needs to listen on IPv6 as well as IPv4. Most do by default, but confirm rather than assuming.
FIREWALL
IPv6 rules are separate from IPv4 rules. A firewall configured only for IPv4 leaves IPv6 completely open, which is a genuine and frequently overlooked exposure.
Configure both, or disable IPv6 entirely if you do not use it.
If the server sends mail over IPv6, it needs IPv6 reverse DNS and an SPF record covering the IPv6 address, or deliverability suffers. Many providers are stricter about IPv6 senders.
WHETHER TO ENABLE IT
If you do not need it, disabling it removes a configuration surface. If you do enable it, configure it as carefully as IPv4.
TESTING
Use an online IPv6 testing tool to confirm what is actually reachable.
