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IPv6 Subnet Calculator

Prefix range, number of addresses and /64 networks, and a list of subnets when you split a delegation.

Runs in your browser – nothing is sent or stored.

IPv6 numbers are too large to work out by hand, and planning is different from IPv4: you think in /48s, /56s and /64s rather than in host counts. Enter a prefix to see its range and size, then split it to get a clean list of networks – ideal for writing an addressing plan or a router configuration.

How to use it

  1. Enter the prefix you received from your ISP or RIR, e.g. 2001:db8:4c00::/48.
  2. Choose what to split it into – usually /64 for LANs, or /56 per site.
  3. Copy the subnets into your plan. Host bits that were set in your input are corrected and flagged.

A simple plan that scales

With a /48 you have 65,536 /64 networks. A readable scheme encodes meaning in the fourth group – for example :01xx: for site 1 and xx10 for VLAN 10 – so 2001:db8:4c00:0110::/64 is “site 1, VLAN 10”. Engineers can read the address and know where it lives.

Don’t go smaller than /64 on a LAN

SLAAC, privacy addresses and many Android devices only work on /64 networks. Use /127 for router-to-router links and /128 for loopbacks; everything with end devices gets a /64.

Frequently asked questions

Why is every LAN a /64?

SLAAC – the way devices give themselves an address – needs 64 bits for the interface ID, so a LAN subnet should always be a /64 even though it holds 18 quintillion addresses.

What prefix should an ISP give a site?

RIPE-690 recommends a /48 for businesses and a /56 for homes, which leaves room for 65,536 or 256 separate /64 networks.

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