Is my IPv6 addressing plan sent to a server?

No. All calculations run in your browser using 128-bit BigInt arithmetic; nothing is transmitted anywhere, so internal prefixes and addressing plans stay on your machine.

Can I enter compressed addresses with double colons?

Yes. Input like 2001:db8::/48 is expanded to the full 8 groups internally, and results are re-compressed for display by collapsing the longest run of zero groups back into ::.

Why is there no usable-hosts figure like in IPv4?

IPv6 has no broadcast address, so no addresses are subtracted from a subnet the way IPv4 reserves network and broadcast addresses. The tool therefore shows the full address count for the prefix.

IPv6 Subnet Calculator

Overview

A CIDR calculator for IPv6. Enter a prefix and it computes the network address, the first and last addresses in the range, the prefix length, and the total number of addresses — using exact 128-bit BigInt math, so even a /0 with its 340-undecillion addresses is computed precisely. Results update as you type, and compressed (::) input is fully supported. Useful when carving a /48 allocation into /64 subnets or verifying route boundaries.

How to use

Enter a value in the IPv6 CIDR field, e.g. 2001:db8::/48 — compressed or fully expanded notation both work.
Read the computed rows (Network, First address, Last address, Prefix, Addresses), which refresh on every keystroke. Invalid input (prefix outside 0–128 or a malformed address) shows an inline error.

Examples

2001:db8::/48 → Last address 2001:db8:0:ffff:ffff:ffff:ffff:ffff, 2^80 addresses.
2001:db8:abcd:12::/64 → Last address 2001:db8:abcd:12:ffff:ffff:ffff:ffff, 18,446,744,073,709,551,616 addresses.
2001:db8:abcd:12:3456::1/64 → Network 2001:db8:abcd:12:: — the calculator masks any host bits you include.

Notes

Everything runs locally in your browser; nothing is uploaded. Output addresses are shown in canonical compressed form, with the longest run of zero groups collapsed to ::.