IPv6 Subnet Calculator
Enter an IPv6 address or prefix to see it expanded and compressed, the first and last address of the prefix, how many addresses and /64 subnets it holds, what kind of address it is and its reverse DNS zone. Then split the prefix into subnets of any longer length and page through them. Everything runs in your browser.
How IPv6 prefixes and subnets work
- An IPv6 address has 128 bits, written as eight groups of four hex digits. A prefix such as 2001:db8:abcd::/48 fixes the first 48 bits and leaves the other 80 bits free (RFC 4291, section 2.3).
- To split a prefix, keep the bits the prefix fixes and count in the extra bits you add. Splitting a /48 into /64s uses the 16 bits in between, so you get 216 = 65,536 subnets, numbered 2001:db8:abcd::/64, 2001:db8:abcd:1::/64, 2001:db8:abcd:2::/64 and so on up to 2001:db8:abcd:ffff::/64.
- For unicast addresses other than those starting with binary 000, the interface identifier is 64 bits long (RFC 4291, section 2.5.1), which is why /64 is the normal size of a LAN subnet.
- IPv6 has no broadcast address, so the first and last address in a subnet are both ordinary addresses of the prefix.
Writing addresses the standard way
RFC 4291 lets you drop leading zeros in a group and replace one run of all-zero groups with a double colon. RFC 5952 narrows that to one recommended form, and the calculator prints it:
- Lower case hex digits and no leading zeros in a group.
- Use the double colon for the longest run of two or more zero groups. If two runs tie, compress the first.
- Never use the double colon for a single zero group, so 2001:db8:0:1:1:1:1:1 stays as written.
Common prefixes
| Prefix | Meaning | Defined in |
|---|---|---|
| 2000::/3 | Global unicast | RFC 4291 |
| fc00::/7 | Unique local addresses (private) | RFC 4193 |
| fe80::/10 | Link-local unicast | RFC 4291 |
| ff00::/8 | Multicast | RFC 4291 |
| 2001:db8::/32 | Documentation examples | RFC 3849 |
| ::1/128 | Loopback | RFC 4291 |
How much space should a site get?
RFC 6177 says the exact size of an end-site assignment is a matter for the operational community, and that a one-size-fits-all /48 is no longer recommended as a single default. It also says a single /128 should no longer be handed out, because a site by definition has multiple subnets and devices.
Study and related tools
IPv6 addressing appears on CompTIA Network+ and Cisco CCNA. Read the IPv6 addressing cheat sheet and IPv4 vs IPv6, try the free Network+ N10-009 practice test, or use the CIDR and VLSM calculator for IPv4 and the free tools page for more.
IPv6 subnet calculator FAQ
How many /64 subnets are in a /48?
65,536, because 64 minus 48 is 16 and 216 is 65,536.
How many addresses are in a /64?
264, which is 18,446,744,073,709,551,616.
Is there a broadcast address in IPv6?
No. IPv6 replaces broadcast with multicast, so every address in a subnet can be assigned to an interface, apart from reserved ones such as the subnet-router anycast address.
What is the reverse DNS zone for a prefix?
The nibbles of the address in reverse order under ip6.arpa (RFC 3596). A prefix whose length is not a multiple of 4 uses the whole nibbles it covers.
Address formats follow RFC 4291 and RFC 5952, special prefixes follow RFC 4193 and RFC 3849 and the end-site guidance is RFC 6177, all checked on rfc-editor.org on 8 November 2026. Results were tested against Python's ipaddress module and worked examples from the RFCs.