IPv6 Subnet Calculator

An IPv6 subnet calculator shows the network address, first and last addresses, and total address count for any IPv6 prefix. It accepts compressed notation like 2001:db8::1 and returns both the RFC 5952 compressed form and the full eight-group expansion, names the address type — global unicast, link-local fe80::/10, unique local fc00::/7, multicast ff00::/8, loopback, unspecified, documentation or IPv4-mapped — and counts how many /64 subnets the block holds. It also derives the ip6.arpa reverse zone, the solicited-node multicast group, and the MAC address behind an EUI-64 interface identifier. It runs entirely on your device.

How to use it

1. Enter an IPv6 address in compressed or full form. A zone suffix like %eth0 and a whole CIDR block like 2001:db8::/48 are both accepted.
2. Set the prefix length — /64 is the standard for a single network.
3. Read the network, address range, compressed and expanded forms, address type, /64 count and reverse-DNS zone.
4. To carve the block up, put the number of subnets you want in Split into subnets; the first eight are listed with their last address.

Examples

• 2001:db8::1/64 → network 2001:db8::/64 containing 2^64 addresses, documentation prefix (RFC 3849).
• 2001:db8:abcd:12::/48 → a site allocation of 2^80 addresses and 65,536 separate /64 subnets.
• fe80::21a:2bff:fe3c:4d5e/64 → link-local, EUI-64 interface identifier, MAC 00:1a:2b:3c:4d:5e.

Frequently asked questions

Why is /64 the standard IPv6 subnet size?

A /64 leaves 64 bits for the interface identifier, which is what stateless address autoconfiguration and several other IPv6 mechanisms expect. Using anything smaller than a /64 on a normal LAN breaks those features, so /64 is the default even though it is an enormous range.

What does :: mean in an IPv6 address?

The double colon replaces one run of consecutive zero groups, shortening the address. So 2001:db8:0:0:0:0:0:1 compresses to 2001:db8::1. It may appear only once in an address, otherwise the expansion would be ambiguous.

How many addresses are in a /48 allocation?

A /48 leaves 80 bits for addressing, which is 2^80 addresses — around 1.2 septillion. In practice, what matters is that a /48 contains 65,536 separate /64 subnets, which is why /48 is a typical site allocation.

Do IPv6 subnets have broadcast addresses?

No. IPv6 removed broadcast entirely and uses multicast instead, so there is no broadcast address to reserve. That's why every address in an IPv6 subnet is usable, unlike IPv4 where two are lost per subnet.

How do I expand a compressed IPv6 address?

Put back the leading zeros in every group so each is four hex digits, then replace the :: with as many 0000 groups as it takes to reach eight in total. 2001:db8::1 expands to 2001:0db8:0000:0000:0000:0000:0000:0001. The calculator shows the compressed and expanded forms side by side, so you can paste either one and read the other.

What is the difference between fe80::, fc00:: and 2000:: addresses?

fe80::/10 is link-local: valid on one link only, generated automatically, never forwarded by a router. fc00::/7 is unique local — the IPv6 answer to RFC 1918 private addressing, with fd00::/8 the half you self-assign using a random 40-bit global ID. 2000::/3 is global unicast, the publicly routable space your ISP or registry allocates from.

What is a solicited-node multicast address?

It is the group ff02::1:ffXX:XXXX formed from the last 24 bits of an address, and every interface joins the one matching each of its own addresses. Neighbour discovery sends its solicitations there instead of to a broadcast, so only the handful of hosts sharing those last 24 bits are interrupted rather than every device on the link.

How do I work out the ip6.arpa reverse DNS zone?

Expand the address, drop the colons to leave 32 hex nibbles, keep one nibble for every four prefix bits, reverse their order and append ip6.arpa. A /64 therefore uses 16 nibbles: 2001:db8::/64 becomes 0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. Prefixes that are not a multiple of four have no single zone and must be delegated another way.