IPv6 Subnet Cheat Sheet
Quick reference for IPv6 prefix lengths, address capacity, and standard allocation sizes. Click any prefix to open it in the interactive calculator.
| Prefix | Total Addresses | /64 Subnets | Common Use |
|---|---|---|---|
| /128 | 20 | — | Single host, loopback (::1) |
| /127 | 21 | — | Point-to-point links (RFC 6164) |
| /126 | 22 | — | Point-to-point links (4 addresses) |
| /112 | 216 | — | 65,536 hosts — rare, but valid |
| /96 | 232 | — | Legacy IPv4-mapped space |
| /80 | 248 | — | 2^48 addresses |
| /64 | 2128 ≈ 3.4×1038 | 1 /64 | Standard LAN — required for SLAAC/EUI-64 |
| /63 | 2128 ≈ 3.4×1038 | 21 | Two /64 subnets |
| /62 | 2128 ≈ 3.4×1038 | 22 | Four /64 subnets |
| /60 | 2128 ≈ 3.4×1038 | 24 | 16 /64 subnets (ISP allocation to home router) |
| /56 | 2128 ≈ 3.4×1038 | 28 | 256 /64 subnets (ISP /56 allocation) |
| /52 | 2128 ≈ 3.4×1038 | 212 | 4,096 /64 subnets |
| /48 | 2128 ≈ 3.4×1038 | 216 | 65,536 /64 subnets — typical site allocation |
| /44 | 2128 ≈ 3.4×1038 | 220 | 1M /64 subnets |
| /40 | 2128 ≈ 3.4×1038 | 224 | 16M /64 subnets |
| /36 | 2128 ≈ 3.4×1038 | 228 | 268M /64 subnets |
| /32 | 2128 ≈ 3.4×1038 | 232 | ISP allocation — 65,536 /48 sites |
| /24 | 2128 ≈ 3.4×1038 | 240 | Large ISP / National registry |
| /20 | 2128 ≈ 3.4×1038 | 244 | Regional allocation |
| /16 | 2128 ≈ 3.4×1038 | 248 | IANA block allocation |
| /12 | 2128 ≈ 3.4×1038 | 252 | ~4096 /24 blocks |
| /8 | 2128 ≈ 3.4×1038 | 256 | Continental or large registry |
| /4 | 2128 ≈ 3.4×1038 | 260 | Major IANA designated block |
| /0 | 2128 ≈ 3.4×1038 | 264 | Default route — all IPv6 traffic |
Why is /64 so important in IPv6?
The /64 prefix is the foundational building block of IPv6 networking. It is the required boundary for Stateless Address Autoconfiguration (SLAAC), where hosts generate their own IP address by combining the /64 network prefix with a 64-bit interface identifier derived from the MAC address (EUI-64). Every LAN segment should be a /64, regardless of how many hosts it contains.
This is one of the biggest mindset shifts from IPv4. In IPv4, you'd carefully calculate the smallest possible subnet to "save" addresses. In IPv6, you give every LAN a full /64 — which alone contains 18.4 quintillion addresses. There is no scarcity.
Standard ISP Allocation Sizes
The Internet community has converged on specific allocation sizes at each tier:
- /48 — Standard allocation to an end-site or organization (65,536 /64 subnets).
- /56 — Common ISP allocation to residential customers (256 /64 subnets).
- /60 — Minimal ISP allocation to home routers (16 /64 subnets, enough for separate VLANs).
- /32 — Standard allocation to an ISP from a Regional Internet Registry (RIR).
Common Reserved IPv6 Prefixes
These prefixes have globally assigned meanings and should never be used for general routing:
::1/128— Loopback (equivalent to 127.0.0.1/8 in IPv4).fe80::/10— Link-local addresses (auto-configured, non-routable).fc00::/7— Unique Local Addresses (ULA), the IPv6 equivalent of RFC 1918 private space.2001:db8::/32— Reserved for documentation and examples.2002::/16— 6to4 tunneling addresses.ff00::/8— Multicast addresses.