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.
/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