breakitdown

What does /56 mean in IPv6?

Answer

An IPv6 /56 leaves 8 bits for subnets before the standard /64 boundary, so it holds 256 /64 subnets. It is the usual size for a residential customer or a small site.

Worked example: 2001:db8:abcd:ff00::/56. Type any block to compare.

Try

2001:db8:abcd:ff00::/56 is 2^72 ≈ 4.7 sextillion addresses: 2001:db8:abcd:ff00:: to 2001:db8:abcd:ffff:ffff:ffff:ffff:ffff. IPv6 has no broadcast address.

20010db8abcdff000000000000000000
56-bit prefix 72 bits for subnets and hosts (each hex digit is 4 bits)
Network
2001:db8:abcd:ff00::
First address
2001:db8:abcd:ff00::
Last address
2001:db8:abcd:ffff:ffff:ffff:ffff:ffff
Addresses
2^72 ≈ 4.7 sextillion
Usable
≈ 4.7 sextillion
Expanded
2001:0db8:abcd:ff00:0000:0000:0000:0000
Range type
Documentation · not routed on the internet
/64 subnets inside
256

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:abcd:ff00::/642001:db8:abcd:ff00::1 – 2001:db8:abcd:ff00:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff01::/642001:db8:abcd:ff01::1 – 2001:db8:abcd:ff01:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff02::/642001:db8:abcd:ff02::1 – 2001:db8:abcd:ff02:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff03::/642001:db8:abcd:ff03::1 – 2001:db8:abcd:ff03:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff04::/642001:db8:abcd:ff04::1 – 2001:db8:abcd:ff04:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff05::/642001:db8:abcd:ff05::1 – 2001:db8:abcd:ff05:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff06::/642001:db8:abcd:ff06::1 – 2001:db8:abcd:ff06:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff07::/642001:db8:abcd:ff07::1 – 2001:db8:abcd:ff07:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff08::/642001:db8:abcd:ff08::1 – 2001:db8:abcd:ff08:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff09::/642001:db8:abcd:ff09::1 – 2001:db8:abcd:ff09:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff0a::/642001:db8:abcd:ff0a::1 – 2001:db8:abcd:ff0a:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff0b::/642001:db8:abcd:ff0b::1 – 2001:db8:abcd:ff0b:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff0c::/642001:db8:abcd:ff0c::1 – 2001:db8:abcd:ff0c:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff0d::/642001:db8:abcd:ff0d::1 – 2001:db8:abcd:ff0d:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff0e::/642001:db8:abcd:ff0e::1 – 2001:db8:abcd:ff0e:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion
2001:db8:abcd:ff0f::/642001:db8:abcd:ff0f::1 – 2001:db8:abcd:ff0f:ffff:ffff:ffff:ffff2^64 ≈ 18.4 quintillion

Is this IP in the range?

Range to CIDR blocks

What to know

ISPs hand a /56 to a home router through DHCPv6 prefix delegation; the router then numbers its LAN, guest Wi-Fi and other VLANs from it, one /64 each. In 2001:db8:abcd:ff00::/56, the subnets are 2001:db8:abcd:ff00::/64 through 2001:db8:abcd:ffff::/64.

RIPE's best current practice for ISPs (RIPE-690) recommends a /56 for residential customers and a /48 for businesses, and recommends that the prefix stays the same over time. RFC 6177 no longer insists on a /48 for every site but says homes should get significantly more than a single /64.

Gotchas

Related explainers

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