breakitdown

What does /64 mean in IPv6?

Answer

An IPv6 /64 fixes the first 64 bits as the network and leaves 64 bits for the interface ID: 18,446,744,073,709,551,616 addresses. It is the standard size for every IPv6 LAN or VLAN, however few devices are on it.

Worked example: 2001:db8:1234:5678::/64. Type any block to compare.

Try

2001:db8:1234:5678::/64 is 2^64 ≈ 18.4 quintillion addresses: 2001:db8:1234:5678:: to 2001:db8:1234:5678:ffff:ffff:ffff:ffff. IPv6 has no broadcast address.

20010db8123456780000000000000000
64-bit prefix 64 bits for subnets and hosts (each hex digit is 4 bits)
Network
2001:db8:1234:5678::
First address
2001:db8:1234:5678::
Last address
2001:db8:1234:5678:ffff:ffff:ffff:ffff
Addresses
2^64 ≈ 18.4 quintillion
Usable
≈ 18.4 quintillion
Expanded
2001:0db8:1234:5678:0000:0000:0000:0000
Range type
Documentation · not routed on the internet

Special-purpose range

Documentation 2001:db8::/32 RFC 3849
Reserved for examples in documentation. Never routed.

Split it into subnets

or

256 subnets, first 16 shown.

SubnetRangeAddresses
2001:db8:1234:5678::/722001:db8:1234:5678::1 – 2001:db8:1234:5678:ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:100::/722001:db8:1234:5678:100::1 – 2001:db8:1234:5678:1ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:200::/722001:db8:1234:5678:200::1 – 2001:db8:1234:5678:2ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:300::/722001:db8:1234:5678:300::1 – 2001:db8:1234:5678:3ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:400::/722001:db8:1234:5678:400::1 – 2001:db8:1234:5678:4ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:500::/722001:db8:1234:5678:500::1 – 2001:db8:1234:5678:5ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:600::/722001:db8:1234:5678:600::1 – 2001:db8:1234:5678:6ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:700::/722001:db8:1234:5678:700::1 – 2001:db8:1234:5678:7ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:800::/722001:db8:1234:5678:800::1 – 2001:db8:1234:5678:8ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:900::/722001:db8:1234:5678:900::1 – 2001:db8:1234:5678:9ff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:a00::/722001:db8:1234:5678:a00::1 – 2001:db8:1234:5678:aff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:b00::/722001:db8:1234:5678:b00::1 – 2001:db8:1234:5678:bff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:c00::/722001:db8:1234:5678:c00::1 – 2001:db8:1234:5678:cff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:d00::/722001:db8:1234:5678:d00::1 – 2001:db8:1234:5678:dff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:e00::/722001:db8:1234:5678:e00::1 – 2001:db8:1234:5678:eff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion
2001:db8:1234:5678:f00::/722001:db8:1234:5678:f00::1 – 2001:db8:1234:5678:fff:ffff:ffff:ffff2^56 ≈ 72.1 quadrillion

Is this IP in the range?

Range to CIDR blocks

What to know

IPv6 addressing (RFC 4291) gives ordinary unicast addresses a 64-bit interface identifier, and stateless address autoconfiguration (SLAAC, RFC 4862) builds each device's address by combining the router's advertised /64 prefix with an interface ID the device chooses itself. With a prefix longer than /64, SLAAC does not work, and Android devices, which do not support DHCPv6 for addresses, get no IPv6 address at all.

The size is not about filling it. A /64 per subnet means you never renumber because a subnet grew, and privacy addresses can pick random interface IDs with no realistic chance of collision.

Gotchas

Related explainers

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