IPv6 Address & Prefix
About the IPv6 Subnet Calculator
IPv6 uses 128-bit addresses written as eight groups of hexadecimal digits. Subnetting in IPv6 works on prefix length rather than dotted subnet masks: a /64 is the standard size for a single LAN segment, a /48 is a common site allocation, and a /56 is typical for residential delegated prefixes. This calculator takes any IPv6 address and prefix and shows the network address, the first and last address in the range, both the compressed and fully expanded notation, and the total number of addresses in the block.
Everything is computed locally in your browser using BigInt arithmetic, so no addresses leave your device. That makes it safe to use with internal addressing plans and production prefixes.
How to Use This Tool
1
Enter any IPv6 address — compressed (
2001:db8::1) or fully expanded forms are both accepted.2
Set the prefix length from /0 to /128; /64 is the standard LAN size and a good default.
3
Read the results: network address, first and last address in the range, both compressed and expanded notation, and the total address count.
4
Use the expanded form when you need to see exactly which of the 128 bits the prefix covers — invaluable when carving a /48 into /64s.
Common Use Cases
- Site addressing plans. Split the /48 your provider delegates into /64 VLANs — one per network segment — and document the ranges before deployment.
- Troubleshooting "same network" issues. Check whether two IPv6 addresses actually fall inside the same prefix when neighbour discovery or routing behaves oddly.
- Firewall and ACL scoping. Confirm exactly which addresses a /56 or /60 rule covers before applying it — expanded notation removes the guesswork.
- Cloud VPC design. Azure and AWS hand out IPv6 blocks in fixed sizes; verify how many /64 subnets fit in your allocation and where each begins.
- Learning IPv6. Watching compressed addresses expand — and seeing why
::can appear only once — teaches more than any diagram.
Common IPv6 Prefix Sizes
- /64. Standard for a single subnet or VLAN. Required for SLAAC autoconfiguration to work.
- /48. Typical site allocation, giving 65,536 /64 subnets for one organisation or location.
- /56. Common residential or small-business delegation, giving 256 /64 subnets.
- /127. Recommended for point-to-point router links (RFC 6164).
Frequently Asked Questions
Why is /64 the standard subnet size when it wastes so many addresses?
SLAAC (stateless address autoconfiguration) requires a 64-bit interface identifier, so anything longer than /64 on a LAN breaks automatic addressing. IPv6 is designed around abundant space — a single /64 holding 18 quintillion addresses is intentional, not waste.
Can I subnet on a boundary that isn't a multiple of 4 bits?
Yes — prefixes work at any bit length, /61 and /63 are valid. But non-nibble boundaries don't align with the hexadecimal digits humans read, which makes plans error-prone. Stick to /48, /52, /56, /60, /64 where you can.
Why does IPv6 not use subnet masks like 255.255.255.0?
IPv6 dropped dotted masks entirely. The prefix length after the slash states how many leading bits identify the network, which is the same idea CIDR brought to IPv4 but without the legacy notation.
Why is /64 the standard LAN size?
SLAAC builds the host portion of an address from 64 bits, so stateless autoconfiguration only works on /64 networks. Unless you have a specific reason, every end-user segment should be a /64.
What does the :: mean in an address?
A double colon compresses the longest run of consecutive zero groups. It can appear only once per address. This tool shows both the compressed and the fully expanded forms so you can copy whichever your platform expects.