IPv6 Subnet Calculator
Calculate an IPv6 subnet from an address and prefix length: the network (prefix) address, first and last addresses, CIDR notation, and the exact total number of addresses — computed with BigInt so even a /0 count is precise.
Input
Any IPv6 address, compressed (2001:db8::) or full. The '::' abbreviation is supported.
The subnet prefix length, 0-128. /64 is the standard IPv6 subnet size.
Output
| Property | Value |
|---|---|
| No data yet | |
Guides
The IPv6 Subnet Calculator takes an IPv6 address and a prefix length and works out everything that defines the subnet around it: the network (prefix) address, the first and last addresses in the range, the CIDR notation, and the exact total number of addresses the block contains. Everything runs in your browser — nothing is sent to a server.
How to use it
- Type an IPv6 address into the address field. Compressed forms with
::(such as2001:db8::) and fully written-out addresses both work. - Drag the Prefix length slider to the size of your subnet, anywhere from
/0to/128. The standard IPv6 subnet is/64, which is the default. - Read the results table. It updates instantly as you change either input.
The Network / Prefix Address is the address with all host bits cleared — the canonical name of the subnet. In IPv6 there is no separate "network" or "broadcast" address the way IPv4 has, so the First Address equals the prefix address and the Last Address is the prefix address with every host bit set. CIDR Notation gives you the block in address/prefix form, ready to paste into a router or firewall rule.
Why the address count needs big integers
A single /64 subnet contains 2^64 = 18,446,744,073,709,551,616 addresses. That is already far larger than a normal JavaScript number can represent exactly (the safe limit is about 2^53), and a /0 spans 2^128 — a 39-digit number. This calculator carries every address as a 128-bit BigInt, so the total is always exact rather than rounded. The result shows both the fully grouped decimal figure and the matching power of two (for example 2^96) so you can sanity-check the magnitude at a glance.
What prefix length should I use?
/64 is the recommended size for a single LAN segment — it is what stateless address autoconfiguration (SLAAC) expects. Providers typically hand out a /48 or /56 to a site, leaving room to carve many /64 subnets beneath it. /128 describes one specific host, and /0 covers the entire IPv6 address space.
Does it support the :: shorthand?
Yes. The :: token stands for one run of all-zero groups and may appear once. 2001:db8:: and 2001:0db8:0000:0000:0000:0000:0000:0000 are treated identically, and outputs are always returned in the shortest RFC 5952 form (lowercase, longest zero run compressed).
Is my data private?
Completely. The calculation is pure client-side arithmetic — the address you enter never leaves your device.
Common IPv6 ranges
2001:db8::/32 is reserved for documentation, fe80::/10 is link-local, and fc00::/7 (commonly fd00::/8) is the unique-local range — the IPv6 equivalent of private IPv4 space.