IPv6 Subnet Calculator

IPv6 planning feels different from IPv4 planning because the question changes. In IPv4 you count hosts and squeeze them into the smallest block. In IPv6 every ordinary LAN gets a /64, and the real planning task is deciding how a larger prefix is divided into /64 networks. This page takes a prefix and shows its range, how many /64 networks it contains and the first subnets in the list.

Prefix sizes you will meet

Registries give a provider a block of /32 or shorter, and providers delegate smaller prefixes to customers. A /48 for a site or business and a /56 for a home are common choices; RFC 6177 advises giving an end site more than a single /64. Within the customer’s prefix, each LAN, VLAN or Wi-Fi network takes one /64. Some features, for example stateless address autoconfiguration, assume the host part is 64 bits, so using a longer prefix on an ordinary LAN breaks them.

What the page calculates

Enter an address or prefix, or pick one of the chips. The result shows the prefix, its network in full 128-bit form, the first and last address, the total number of addresses (as an exact integer and a power of two), the count of /64 subnets inside, the split between network bits and host bits, and the kind of address, for instance Documentation for 2001:db8::/32. A table then lists the first 16 /64 subnets of the prefix in both short and full notation. The lists are capped because a /32 holds more than four billion /64 networks.

Reading the numbers

Subtract the prefix length from 64 to count /64 networks as a power of two: a /48 is 2¹⁶ = 65,536, a /56 is 2⁸ = 256, a /60 is 2⁴ = 16 and a /64 is exactly one. Add one hex digit (four bits) to the prefix and the number of subnets multiplies by sixteen. Prefixes that are multiples of four bits line up with a hexadecimal digit, which makes them easy to read in an address; a /52 or /56 allocation changes only the digit after the fourth colon.

A worked example

Choose the chip for 2001:db8:1234::/48. The page shows the network in full as 2001:0db8:1234:0000:0000:0000:0000:0000, the first address 2001:db8:1234::, the last 2001:db8:1234:ffff:ffff:ffff:ffff:ffff and a total of 1,208,925,819,614,629,174,706,176 addresses, which is 2⁸⁰. It reports 65,536 /64 subnets, with 48 network bits and 80 host bits, and labels the kind of address as Documentation. The table starts with 2001:db8:1234::/64, then 2001:db8:1234:1::/64, 2001:db8:1234:2::/64 and continues to 2001:db8:1234:f::/64.

Limits

The calculator performs arithmetic and does not query registries, check that a prefix has been assigned to you, or test routing. It lists only the first 16 subnets of a prefix. For IPv4 blocks use the Subnet Calculator, which also accepts IPv6 but is aimed at the host-count questions IPv4 raises.

A readable numbering plan

Because the fourth group of a /48 is a whole 16 bits, it makes a convenient label for each network. A site with the prefix 2001:db8:1234::/48 might number servers as 2001:db8:1234:10::/64, staff as 2001:db8:1234:20::/64 and guests as 2001:db8:1234:30::/64, with the VLAN number or a department code in that group. The prefixes then read like a table and are easy to summarise: 2001:db8:1234:10::/60 covers sixteen networks from :10 to :1f. Sketch the plan here, then copy the first subnets into your router configuration.

Related tools

• Subnet Calculator handles IPv4 blocks with broadcast and host counts.
• IPv6 Expander shows any address in the full 128-bit form.
• IPv6 Compressor shortens addresses to the canonical form.
• Reverse DNS Zone Generator builds ip6.arpa zones for a prefix.