IPv6 subnetting, without the guesswork.
Enter an IPv6 address and prefix length from /0 to /128. The calculator finds the canonical network prefix, expands the address, counts host/interface bits and visualizes how much of the 128-bit address belongs to the network.
How an IPv6 subnet calculator works
IPv6 addresses contain 128 bits, four times as many as IPv4. They are written as eight hexadecimal groups separated by colons. A prefix length such as /64 tells you how many leading bits identify the network. The remaining bits are available for the interface identifier or other addressing structure inside that prefix.
This calculator expands compressed IPv6 notation, applies the selected prefix bit-by-bit, clears the host portion to zero and returns the canonical network prefix. All calculation happens locally in your browser. It does not submit the IPv6 address to a backend service.
Why /64 is so common in IPv6
For ordinary IPv6 LANs, /64 is the standard subnet size. It leaves 64 interface-identifier bits, which is the structure assumed by Stateless Address Autoconfiguration (SLAAC) in normal deployments. A /64 contains 264 possible addresses—18,446,744,073,709,551,616—so IPv6 planning is usually about hierarchical prefix allocation rather than conserving individual host addresses.
Common IPv6 prefix lengths
A /32 is commonly seen as an ISP or large organizational allocation boundary, /48 is often used for a site, /56 may be delegated to a smaller customer site, /64 is the normal LAN subnet size, and /128 identifies one exact address. Real allocation policies vary by provider and network design, so the prefix should be chosen according to the actual routing and addressing plan.
IPv6 does not use dotted-decimal subnet masks
IPv4 administrators may be familiar with masks such as 255.255.255.0. IPv6 normally expresses the boundary directly as a prefix length. For example, 2001:db8:1234:5678::/64 means the first 64 bits identify the network prefix. There is no need to convert that to a dotted-decimal mask.
Compressed and expanded IPv6 notation
IPv6 allows leading zeroes in each hexadecimal group to be omitted, and one consecutive run of zero groups can be replaced with ::. The address 2001:db8:1234:5678::1 is therefore a compact form of 2001:0db8:1234:5678:0000:0000:0000:0001. The calculator displays the expanded network form so prefix boundaries are easier to inspect.
IPv4 vs IPv6 subnetting
Both protocols use CIDR prefix lengths, but the practical goals differ. IPv4 subnetting often focuses on conserving a limited address pool and calculating network/broadcast addresses. IPv6 has an enormous address space and does not use IPv4-style broadcast addresses. IPv6 design instead emphasizes clean hierarchical allocation, route aggregation and consistent /64 LAN boundaries.
Frequently asked questions
What is an IPv6 subnet calculator?
An IPv6 subnet calculator takes an IPv6 address and prefix length, then identifies the canonical network prefix, host/interface bit count, expanded address form and address-space size.
What is the normal IPv6 subnet size?
A /64 is the standard subnet size for most IPv6 LANs and is required for SLAAC in typical deployments. It leaves 64 bits for the interface identifier.
How many addresses are in an IPv6 /64?
An IPv6 /64 contains 2^64 addresses, or 18,446,744,073,709,551,616 possible addresses.
Does IPv6 use subnet masks like 255.255.255.0?
IPv6 normally uses prefix-length notation such as /48 or /64 instead of dotted-decimal subnet masks.
What does /128 mean in IPv6?
/128 identifies exactly one IPv6 address because all 128 bits are part of the prefix.
Related tools
For IPv4, use the main subnet calculator, CIDR calculator, subnet mask calculator or subnetting cheat sheet.