IPv6 Subnet Calculator
Enter an IPv6 network in CIDR notation and get the network prefix, first and last address, the exact total number of addresses, and how many /64 subnets it holds. Everything is computed with 128-bit BigInt math in your browser.
2001:db8::/32, fd00::/48 or 2001:db8:abcd:12::/64
An IPv6 subnet is defined by a prefix — the number of leading bits that are fixed for every address in the network. Written after a slash (for example /32 or /64), the prefix length works exactly like a CIDR prefix in IPv4, just over 128 bits instead of 32. This calculator masks your address down to the prefix, then reports the first address (the network/prefix address), the last address, and the size of the block.
Because IPv6 blocks are astronomically large, raw address counts are rarely useful on their own. The number that matters day to day is how many /64 subnets a block provides, since a /64 is the standard size for a single link. A /48 — a common site allocation — contains 65,536 /64 networks, which is why registries often hand a whole /48 or /56 to a single customer.
Almost every ordinary IPv6 LAN is a /64. SLAAC, privacy addresses and Modified EUI-64 all assume a 64-bit interface identifier, so subnetting below /64 is unusual and can break autoconfiguration. Plan your hierarchy in /64, /56 and /48 chunks rather than counting individual addresses.
All of the heavy lifting uses JavaScript's BigInt, so the 128-bit values are exact rather than rounded floating-point approximations. Nothing you enter leaves your device, which makes the tool safe for planning internal or production addressing. If you are new to how prefixes and masks work, the guide on what a subnet mask is explains the underlying idea, and IPv4 vs IPv6 covers why the address space jumped from 32 to 128 bits in the first place.
Need related conversions? Use the IPv6 Expand & Compress tool to normalise an address, or the IPv6 to Binary converter to see the bits behind a prefix boundary.
Frequently asked questions
How many /64 subnets are in a /48?
A /48 contains 2^(64−48) = 2^16 = 65,536 individual /64 subnets. Because the standard host subnet in IPv6 is a /64, network planners usually count how many /64s a block holds rather than how many individual addresses.
Why is the smallest recommended IPv6 subnet a /64?
Stateless Address Autoconfiguration (SLAAC) and Modified EUI-64 both assume a 64-bit interface identifier, so most on-link IPv6 subnets are /64. Using a longer prefix (a smaller subnet) can break SLAAC, privacy addresses and some neighbour-discovery behaviour.
How many total addresses does an IPv6 prefix contain?
An IPv6 prefix of length p contains 2^(128−p) addresses. A /64, for example, holds 2^64 ≈ 1.8 × 10¹⁹ addresses — far more than the entire IPv4 Internet. This tool uses JavaScript BigInt so the 128-bit numbers are exact.
Does this IPv6 subnet calculator send my data anywhere?
No. All parsing and 128-bit BigInt math run entirely in your browser. Nothing you type is uploaded, logged or sent to a server, so it is safe to use with internal or production addressing plans.
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