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IPv6 /56 prefix

A /56 leaves 72 bits for subnets and hosts: 4,722,366,482,869,645,213,696 addresses, or 256 /64 subnets.

IPv6 /56 at a glance

Mask
ffff:ffff:ffff:ff00::
Mask (expanded)
ffff:ffff:ffff:ff00:0000:0000:0000:0000
Host bits
72
Addresses
4,722,366,482,869,645,213,696
Nibble-aligned
Yes, the boundary falls on a hex digit
Reverse DNS zone
0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa

Typical uses of a /56

A /56 is what many ISPs delegate to a home or small business via DHCPv6 prefix delegation, giving 256 /64 LANs. AWS also assigns an Amazon-provided IPv6 block of /56 to each VPC, out of which you cut /64 subnets.

Example: 2001:db8::/56

First address
2001:db8::
Last address
2001:db8:0:ff:ffff:ffff:ffff:ffff
Last (expanded)
2001:0db8:0000:00ff:ffff:ffff:ffff:ffff
Next /56
2001:db8:0:100::/56

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 /56 for an address

What a /56 contains

Smaller prefixHow many
/6016
/64256
/11272,057,594,037,927,936
/1272,361,183,241,434,822,606,848
/1284,722,366,482,869,645,213,696

How many /56s fit

In aNumber of /56s
/3216,777,216
/48256

Cloud support for /56

PlatformIPv6 subnet size/56?
AWS VPC/44 to /64 in steps of /4Yes
Azure VNetexactly /64No
Google Cloud VPC/64 (from the VPC's /48)No