HTTPfy

IPv6 /64 prefix

A /64 leaves 64 bits for subnets and hosts: 18,446,744,073,709,551,616 addresses.

IPv6 /64 at a glance

Mask
ffff:ffff:ffff:ffff::
Mask (expanded)
ffff:ffff:ffff:ffff:0000:0000:0000:0000
Host bits
64
Addresses
18,446,744,073,709,551,616
Nibble-aligned
Yes, the boundary falls on a hex digit
Reverse DNS zone
0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa

Typical uses of a /64

The /64 is the standard IPv6 subnet. Interface identifiers are 64 bits (RFC 4291) and SLAAC only works on a /64, so every LAN, VLAN and cloud subnet should be one. AWS, Azure (which requires exactly /64) and Google Cloud all use /64 subnets.

Example: 2001:db8::/64

First address
2001:db8::
Last address
2001:db8::ffff:ffff:ffff:ffff
Last (expanded)
2001:0db8:0000:0000:ffff:ffff:ffff:ffff
Next /64
2001:db8:0:1::/64

Uses the documentation prefix 2001:db8::/32 (RFC 3849). IPv6 has no broadcast address, so every address in the range is assignable, except where a platform reserves some.

Find the /64 for an address

What a /64 contains

Smaller prefixHow many
/112281,474,976,710,656
/1279,223,372,036,854,775,808
/12818,446,744,073,709,551,616

How many /64s fit

In aNumber of /64s
/324,294,967,296
/4865,536
/56256

Cloud support for /64

PlatformIPv6 subnet size/64?
AWS VPC/44 to /64 in steps of /4Yes
Azure VNetexactly /64Yes
Google Cloud VPC/64 (from the VPC's /48)Yes