IPv6 Subnet Calculator

What is IPv6 subnetting?

IPv6 is the newer, much larger address space that's gradually replacing IPv4 — instead of 32 bits it uses 128, written as 8 groups of hex digits like 2001:db8:1234::1. Subnetting an IPv6 address means splitting that huge address space into smaller networks using a prefix (like /64) that marks how many leading bits are the fixed "network" portion. It's the same idea as IPv4 CIDR, just with a far bigger number space and no broadcast address to worry about.

How to use IPv6 Subnet Calculator

1. Enter an address in IPv6 address (or click Example for a sample).
2. Enter a prefix length in Prefix — e.g. /64.
3. Read the compressed/expanded forms, network prefix, address range and interface ID under Results.
4. Click Copy to copy the full result.

How IPv6 subnetting works

An IPv6 address is a 128-bit number, written as 8 groups of 4 hex digits. A prefix length (/64) works exactly like IPv4 CIDR, just out of 128 bits instead of 32: it fixes how many leading bits are the network portion, leaving the rest as host bits. Unlike IPv4, IPv6 has no broadcast address — every address in the block, including the all-zeros one, is a valid host:

2001:db8:1234::1/64
→ Network prefix:  2001:db8:1234::/64
→ Address range:   2001:db8:1234:: – 2001:db8:1234:0:ffff:ffff:ffff:ffff
→ Interface ID:    0000:0000:0000:0001
→ Total addresses: 18,446,744,073,709,551,616  (2^64)

RFC 5952 defines the canonical text form used above: hex digits are lowercase, leading zeros are dropped from each group, and :: replaces the single longest run of consecutive all-zero groups — used at most once per address, and never for a lone zero group (so 2001:db8:0:1::1 keeps its single 0 written out). This tool builds the address as a real 128-bit integer (JavaScript's native numbers only give you 53 safe bits, nowhere near enough) and runs every calculation — network, range, and the interface split — with exact big-integer arithmetic, the same category of bug that has bitten IPv4-only tools ported carelessly to IPv6.

Frequently asked questions

What does an IPv6 prefix length mean?

An IPv6 prefix length (e.g. /64) is exactly like IPv4 CIDR notation but out of 128 bits instead of 32: it says how many leading bits of the address are the fixed network portion. /64 is the conventional boundary for a single LAN segment — the remaining 64 bits are the interface ID.

Why doesn't IPv6 have a broadcast address?

IPv6 removed broadcast entirely (RFC 4291) in favor of multicast. Every address in a subnet, including the all-zeros network address, is a valid unicast host address — there's no address to subtract for broadcast like IPv4's .255, so a /64 has 2^64 usable addresses, not 2^64 minus 2.

What's the difference between compressed and expanded notation?

Expanded notation writes all 8 groups of 4 hex digits with no shortcuts. Compressed notation (RFC 5952) is the canonical form: leading zeros are dropped from each group, and "::" replaces the single longest run of consecutive all-zero groups — used once per address, and never for just one zero group.

What's the difference between link-local, unique local, and global unicast addresses?

Link-local (fe80::/10) never leaves the local network segment and is auto-assigned on every interface. Unique local (fc00::/7, RFC 4193) is IPv6's equivalent of private IPv4 ranges — routable within an organization but not on the public internet. Global unicast (2000::/3) is publicly routable, assigned by IANA down to individual networks.