IPv6 Prefix Cheat Sheet
Quick reference for IPv6 prefix lengths — address counts, /64 subnet counts, typical use — plus the common special-purpose prefixes.
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Prefix lengths
| Prefix | Addresses | /64 subnets | Typical use | Reference |
|---|---|---|---|---|
| /0 | 2^128 | 2^64 | Default route / all addresses | — |
| /3 | 2^125 | 2^61 | Global unicast space (2000::/3) | RFC 4291 |
| /12 | 2^116 | 2^52 | IANA block delegated to a regional registry | RIR policy |
| /20 | 2^108 | 2^44 | Large registry allocation | RIR policy |
| /29 | 2^99 | 2^35 | Common initial allocation to an LIR | RIR policy |
| /32 | 2^96 | 2^32 | Typical ISP / LIR allocation | RIR policy |
| /40 | 2^88 | 2^24 | Large ISP allocation | RIR policy |
| /44 | 2^84 | 2^20 | Typical ISP / LIR allocation | RIR policy |
| /48 | 2^80 | 2^16 | Common site / enterprise assignment | RIR / ISP policy |
| /52 | 2^76 | 2^12 | Site subdivided into building / department blocks | Operational practice |
| /56 | 2^72 | 2^8 | Common residential / small-site assignment | ISP policy |
| /60 | 2^68 | 2^4 | Common residential / small-site assignment | ISP policy |
| /64 | 2^64 | 1 | Typical /64 subnet and 64-bit IID boundary for most IPv6 unicast links. Addressing architecture: RFC 4291; SLAAC: RFC 4862 | RFC 4291 |
| /96 | 2^32 | within a /64 | Point-to-point or infrastructure links | Operational practice |
| /112 | 2^16 | within a /64 | Point-to-point or infrastructure links | Operational practice |
| /120 | 2^8 | within a /64 | Point-to-point or infrastructure links | Operational practice |
| /126 | 2^2 | within a /64 | Inter-router link (older practice) | Operational practice |
| /127 | 2^1 | within a /64 | Inter-router point-to-point link (RFC 6164) | RFC 6164 |
| /128 | 1 | within a /64 | Single host, loopback, or anycast address | RFC 4291 |
Special-purpose prefixes
| Block | Name | Reference |
|---|---|---|
| ::/0 | Default route | RFC 4291 |
| ::/128 | Unspecified address | RFC 4291 |
| ::1/128 | Loopback address | RFC 4291 |
| ::ffff:0:0/96 | IPv4-mapped IPv6 address | RFC 4291 |
| 64:ff9b::/96 | NAT64 well-known prefix | RFC 6052 |
| 64:ff9b:1::/48 | NAT64 local-use prefix | RFC 8215 |
| 100::/64 | Discard-only address block | RFC 6666 |
| 2000::/3 | Global unicast | RFC 4291 |
| 2001::/32 | Teredo tunneling | RFC 4380 |
| 2001:2::/48 | Benchmarking | RFC 5180 |
| 2001:20::/28 | ORCHIDv2 | RFC 7343 |
| 2001:db8::/32 | Documentation | RFC 3849 |
| 3fff::/20 | Documentation | RFC 9637 |
| 2002::/16 | 6to4 | RFC 3056 |
| fc00::/7 | Unique local address (ULA) | RFC 4193 |
| fe80::/10 | Link-local unicast | RFC 4291 |
| ff00::/8 | Multicast | RFC 4291 |
Assignment and allocation sizes (/29, /32, /48, /56, /60) are RIR / ISP policy or operational practice, not RFC requirements — RFC 6177 deliberately does not define a fixed end-site size. Only /64 and /127 are fixed by a standard. The special-purpose table is a curated subset, not the full IANA registry.
How it works
The first table lists common IPv6 prefix lengths from /0 to /128 with the number of addresses (as a power of two), the number of /64 subnets, and the operational context. Only a few lengths are fixed by a standard: /64 is the typical subnet and 64-bit interface-identifier boundary for most IPv6 unicast links (addressing architecture, RFC 4291; SLAAC itself is RFC 4862 and does not fix the length), and /127 inter-router links follow RFC 6164. Every other length here — /32 or /29 for an ISP, /48 for a site, /56 or /60 for a home — is an allocation or assignment size set by Regional Internet Registry or ISP policy, or by operational practice, and it varies between registries and providers. RFC 6177 (which obsoletes RFC 3177) is often cited here, but it does the opposite of prescribing a size: it withdrew the earlier fixed "/48 to every end site" recommendation and left the choice to the operational community, so this table treats it as the reason there is no single standard size rather than as a source for one. The second table is a curated subset of the IANA IPv6 Special-Purpose Address Registry and the addressing architecture: loopback, unspecified, link-local, unique-local, multicast, documentation, NAT64, and more. This is a static reference; use the IPv6 CIDR Calculator or IPv6 Address Type tool for a specific address.