Plan IPv6 networks with precision.

* This IPv6 Subnet Calculator supports subnet prefixes longer than /64—such as /65 through /128—for specialized, advanced network designs. These prefix lengths should be used only by qualified network professionals who understand their operational, hardware, and compatibility implications.

Although IPv6 routing permits prefixes of any length through /128, a /64 remains the standard subnet size for most IPv6 LANs. Prefixes longer than /64 can prevent IPv6 Stateless Address Autoconfiguration (SLAAC) from operating and may be incompatible with operating systems, applications, network appliances, and protocols or features that assume a 64-bit interface identifier. See RFC 7421, RFC 7608, and RFC 9663.

Using longer-than-/64 prefixes can also substantially increase the number of subnet routes, host routes, neighbor entries, and access-control entries required by routers and switches. Many hardware platforms store this forwarding information in finite TCAM or other limited hardware tables. Exceeding those limits may cause routes or neighbors to fail installation in hardware, traffic to be processed by the CPU, degraded performance, packet loss, or loss of connectivity.

Before deploying a prefix longer than /64, verify the design against the documented IPv6 route, neighbor, ACL, and TCAM capacities of every affected device, and test all required hosts, applications, and protocols in a controlled environment. Point-to-point links, loopbacks, security zones, and other engineered use cases may justify longer prefixes, but they should not be treated as a general replacement for /64 LAN subnets. The calculator provides mathematical addressing results only. It does not determine whether a proposed prefix is supported by your equipment, software, or network design. Use of its results is at your own risk.