IPv6 Subnet Calculator
Expand and compress IPv6 addresses, calculate subnet ranges and counts from a prefix, and plan an allocation using the /64 and /48 conventions that IPv6 addressing assumes.
Last reviewed by the Radiatus Cloud team
Need this monitored 24/7?
Radiatus delivers managed cloud & network operations.
IPv6 subnetting does not conserve addresses
Every habit from IPv4 addressing is wrong in IPv6, because the scarcity that drove them does not exist. A single /64 contains eighteen quintillion addresses, which is more than the entire IPv4 internet squared, and it is the standard size for every subnet regardless of how many hosts are on it. A point to point link gets a /64. A wireless network with four devices gets a /64. Subnetting more finely than /64 breaks stateless address autoconfiguration and the standards that depend on it, so the address saving buys a broken network.
The boundaries that matter
A /48 is the conventional allocation to a site, giving 65,536 subnets of /64 each. A /56 is common for residential connections, giving 256 subnets. A /64 is one subnet. Those three numbers are the whole plan for most organisations: take the /48, use the sixteen subnet bits to encode location and function in a way that reads clearly in hexadecimal, and never think about host counts again.
Compression rules are stricter than they look
The textual representation has rules that most hand written addresses violate. Leading zeros in a group are removed, the longest run of zero groups is replaced by a double colon, that substitution may appear only once, and where two runs are equal length the first is chosen. Hexadecimal digits should be lower case. Two addresses that look different are frequently the same address written by people following different subsets of these rules.
Related tools
- Port Scanner — Check which common ports respond on a host and understand what each exposed service means. Scan only systems you are authorised to test.
- DNS Misconfiguration — Analyze DNS records (TXT, SPF) for security gaps.
- TLS Compat Tester — Check client-server compatibility for TLS versions.
- Firewall Rule Risk — Evaluate firewall rules for over-permissive exposure.
Frequently Asked Questions
Why is /64 the standard subnet size?
Because stateless address autoconfiguration uses the lower 64 bits to derive an address from the interface identifier, and several other mechanisms assume that split. Using a longer prefix on a LAN breaks SLAAC, and the address space is large enough that there is nothing to gain.
How many addresses does a /64 hold?
About 18.4 quintillion, which is 2 to the power 64. That is roughly four billion times the entire IPv4 address space, in one subnet. Host counts stop being a design input in IPv6.
What prefix should I request from my ISP?
A /48 for a business site, which gives 65,536 subnets, or a /56 for a home connection giving 256. An ISP offering a single /64 to a site is not providing enough to subnet at all and is worth pushing back on.
What are the compression rules?
Drop leading zeros in each group, replace the longest run of consecutive zero groups with a double colon, use that substitution at most once, and prefer the first run when two are equally long. Use lower case hexadecimal. The tool shows both the compressed and fully expanded forms.
What is fe80:: for?
It is the link local prefix, present on every IPv6 interface automatically and valid only on that link. It is used for neighbour discovery and router advertisement, and it is why an interface with no global address still shows an IPv6 address.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter an IPv6 prefix to see its range, subnet capacity and the standard allocation boundaries.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
Related Tools
Port Scanner
NetworkCheck which common ports respond on a host and understand what each exposed service means. Scan only systems you are authorised to test.
DNS Misconfiguration
NetworkAnalyze DNS records (TXT, SPF) for security gaps.
TLS Compat Tester
NetworkCheck client-server compatibility for TLS versions.