Network · IPv6 planning

IPv6 subnet calculator.

Normalize any IPv6 address and prefix, find its exact range, then split the block into a longer child prefix such as /64.

No broadcast address: IPv6 uses multicast instead. This calculator shows the mathematical first and last addresses in the prefix.

Calculate a prefix

Host bits are allowed and will be normalized to the network prefix.
Must be the same as or longer than the input prefix.

Result

Enter an IPv6 prefix and press Calculate prefix.

How IPv6 prefix math works

RFC 4291 defines an IPv6 address as 128 bits. A prefix length n fixes the leftmost n bits and leaves 128 minus n bits inside the prefix. The first address has all remaining bits zero; the last has them all one. A /64 therefore contains 264 addresses.

To split a /48 into /64s, the calculator uses 16 more prefix bits: 216, or 65,536 child prefixes. RFC 7421 explains that IPv6 routing supports variable-length prefixes through /128; /64 is the normal subnet size for many unicast links and SLAAC, but it is not the only prefix routing can represent.

Specifications: RFC 4291 sections 2 and 2.3, RFC 5952 for canonical text, and RFC 7421 for prefix-length context.

Address-plan arithmetic

This does not reserve, assign, route, scan, or test any prefix. Confirm your provider allocation and equipment requirements before changing a live network.