IPv6 addressing is simple once you stop counting hosts and start counting /64s. This calculator shows the prefix boundaries, how many /64 LANs a prefix holds, and the reverse name you need for PTR records.
Table of Contents
Planning IPv6 on servers
Give every LAN or VLAN a /64. A single server is usually assigned a /64 by the provider; hosting companies routing prefixes to customers commonly delegate a /56 or /48. Always put the IPv6 address in AAAA records and set a PTR if the server sends mail over IPv6.
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IPv6 subnet calculator at a glance
Official documentation: Windows Server documentation, AlmaLinux wiki, Linux man pages.
Related guides: Troubleshoot MTU, fragmentation and slow VPN throughput · Configure DHCP failover between two Windows Servers · Install OPNsense 26.7 and set up basic firewall and NAT rules.
Frequently asked questions
Why should a LAN be a /64?
SLAAC and many IPv6 features assume 64-bit interface identifiers; smaller subnets break autoconfiguration.
How many /64s are in a /48?
65,536.
What is 2001:db8::/32?
The documentation prefix — safe to use in examples, never routed on the internet.