# IPv6 /64 Prefix – The SLAAC Standard Subnet Size

> An IPv6 /64 prefix contains 18,446,744,073,709,551,616 addresses. Mask math, examples, typical roles and FAQs for planning IPv6 networks.

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# IPv6 /64 Prefix ReferenceIPv6

The atomic unit of IPv6: every normal subnet, whether a home LAN, an office floor, or a VM network, is a /64 holding 18,446,744,073,709,551,616 addresses. The number sounds absurd until you learn why: the bottom 64 bits exist purely for interface identifiers.

An IPv6 /64 prefix contains 18,446,744,073,709,551,616 addresses. It is exactly one /64 — the subnet size SLAAC requires.

## /64 facts at a glance

- **Total addresses:** 18,446,744,073,709,551,616 (264)
- **/64 subnets inside:** 1 — this is the standard subnet itself
- **Host/interface bits:** 64
- **Example network:** 2001:db8::/64
- **First address:** 2001:db8::
- **Last address:** 2001:db8::ffff:ffff:ffff:ffff
- **Typical role:** Standard LAN/VLAN subnet — required for SLAAC

## A /64 in documentation space

Using `2001:db8::/32` (RFC 3849 documentation space) aligned to its own boundary, a /64 occupies:

| Block | First address | Last address |
| --- | --- | --- |
| 2001:db8::/64 | 2001:db8:: | 2001:db8::ffff:ffff:ffff:ffff |

## How an IPv6 /64 prefix works

A /64 prefix pins the first 64 bits of a 128-bit address, leaving 64 bits free. That free portion holds 264 = 18,446,744,073,709,551,616 distinct addresses, and every such block begins at a multiple of that size. All 128 minus 64 remaining bits belong to the interface identifier, which is precisely what SLAAC expects.

## Where /64 is used

- Every SLAAC-speaking LAN: hosts assemble their own addresses inside the /64.
- The standard subnet handed to each VLAN in enterprise designs.
- The delegation unit that ISPs multiply out to homes and sites.

RFC 4291 fixes the interface identifier at 64 bits, which is what makes /64 the practical floor for any subnet expecting autoconfiguration.

SLAAC (RFC 4862) assumes a /64. On longer prefixes, hosts must fall back to DHCPv6 or manual configuration instead of self-assembling addresses.

## Frequently asked questions

**How many addresses are in a /64?**

18,446,744,073,709,551,616, which is 2^64. That single subnet holds more addresses than the entire IPv4 address space by a factor of four billion.

**Why is /64 the smallest recommended IPv6 subnet?**

Because the interface ID is defined as 64 bits (RFC 4291), and SLAAC plus EUI-64 autoconfiguration only work when the host portion has those full 64 bits available.

**What is EUI-64?**

A method hosts use to build an interface ID from the 48-bit MAC address: FFFE is spliced into the middle and the universal/local bit is flipped, yielding a stable 64-bit identifier.

**Should I try to conserve IPv6 subnets like IPv4?**

No. Waste was designed in deliberately: give every segment a /64 and plan hierarchically rather than squeezing hosts together.

## Open this prefix in the calculator

[Open 2001:db8::/64 in the IPv6 calculator](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F64)

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← IPv6 /60 prefix
IPv6 /127 prefix →

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