How IPv6 subnet calculations work
An IPv6 address has 128 bits. In 2001:db8:1234::/48, the /48 fixes the first 48 bits and leaves 80 bits to vary. A prefix of length p therefore contains 2^(128 − p) addresses. A longer prefix describes a smaller range.
You can enter an address anywhere inside a subnet. For example, 2001:db8:1234:5678::42/64 belongs to 2001:db8:1234:5678::/64. The calculator clears the bits after the prefix to find the first address, then sets those bits to find the last. It shows when those host bits were cleared.
Split a /48 or /56 into /64 subnets
Choose a child prefix longer than the parent to divide a block into equal parts. Splitting a prefix of length p into children of length c gives 2^(c − p) children. Choosing the same length returns the original block.
| Parent | Child prefix | Child subnets |
|---|---|---|
2001:db8:1234::/48 | /64 | 65,536 |
2001:db8:1234:5600::/56 | /64 | 256 |
2001:db8:1234:5678::/64 | /64 | 1 |
For the first example, the /64s start at 2001:db8:1234::/64 and end at 2001:db8:1234:ffff::/64. These documentation addresses are examples, not an allocation you can use on the internet.
Address counts and usable networks
A /64 contains exactly 18,446,744,073,709,551,616 addresses; a /127 contains two and a /128 contains one. IPv6 does not use IPv4’s broadcast address convention. Actual assignments still depend on address type, reserved uses, equipment and network design.
A /64 is the usual LAN subnet size for stateless address autoconfiguration (SLAAC) with a 64-bit interface identifier. The ability to calculate another prefix does not mean it suits that LAN. RFC 4862 describes the prefix and interface identifier requirements. The /127 guidance in RFC 6164 specifically concerns inter-router point-to-point links.
Why does my IPv6 address look different?
Leading zeros and letter case can change without changing an address. The normalized view uses lowercase and compresses the longest run of zero groups; the expanded view shows all eight groups. These hexadecimal formatting rules follow RFC 5952. Embedded IPv4 notation is displayed as hexadecimal IPv6. Prefix notation and the 128-bit address model are described in RFC 4291.
This calculator does not check allocation, routing or reachability. Use the IP address range checker to identify special ranges, or read the IPv4 and IPv6 guide. To check your connection instead, run the IPv6 test.