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IPv6 /56 subnet: room for 256 LANs

By TechCompare · Updated · How we check

A /56 subnet holds 256 /64 networks, enough for several separate LANs at a home or small site.

RFC 6177 says registries have revised their end site policy to encourage smaller /56 blocks, and RIPE-690 says some operators give a /56 to residential customers. That makes it a common size for a home or small site. A /56 holds 256 /64 networks, enough for several separate LANs under one allocation.

Prefix
/56
72 host bits
Total addresses
4.72 sextillion
2^72
/64 networks
256
Each one a LAN
A /56 on the 2001:db8:: example prefix, pre-loaded. Swap in your own prefix or step the length up and down.
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How this is calculated

The /56 leaves 72 host bits below its prefix. The 8 bits between the /56 and the /64 are the subnet ID, and they number the LANs inside the block. Each /64 keeps the 64-bit interface ID that RFC 4291 defines for global unicast addresses, so every LAN has the same address layout as any other. A router or firewall handles traffic between them, just as it would for separate IPv4 subnets.

Verdict

Use the /56 when one allocation has to cover several separate networks, such as family devices, guest Wi-Fi, IoT devices and a lab. Give each one its own /64, since RIPE-690 says you can't assign less than a /64 to a LAN segment. RIPE-690 also says assigning prefixes longer than /56 is strongly discouraged. The block runs from 2001:db8:: to 2001:db8:0:ff:ffff:ffff:ffff:ffff, and the 256 /64s inside it are the room you plan with.

More Subnet scenarios

/24 subnet (255.255.255.0)
A /24 subnet, written as 255.255.255.0 in dotted-decimal notation, provides 256 total addresses with 254 usable for hosts (one reserved for the network address and one for broadcast).
View details
/16 subnet (255.255.0.0)
A /16 subnet, written as 255.255.0.0, provides 65,536 total addresses with 65,534 usable for hosts.
View details
/28 subnet (255.255.255.240)
A /28 subnet, written as 255.255.255.240, provides 16 total addresses with 14 usable on a conventional IPv4 subnet.
View details

Related guides

IPv6 /64 subnet
One LAN, the 64-bit boundary and the address math behind it.
View guide
IPv6 /48 subnet
The business block that holds /56 blocks for many sites.
View guide
How many /64s fit in a /48
The worked count for a /48 and a /56, with the bit math.
View guide
IPv4 vs IPv6 comparison
How IPv6 addressing differs from the IPv4 subnets you know.
View guide
/24 subnet
The IPv4 counterpart for a home or small office network.
View guide

Frequently asked questions

Is a /56 the same as a home network?
Not exactly. The /56 is the allocation a provider hands out, and a home network is one or more /64s carved from it. RIPE-690 says some operators give a /56 to residential customers, but the size a provider gives depends on its policy.
Why not give a home a single /64?
A single /64 gives the home one LAN and no room to split it. A /56 leaves room for 256 /64s, so guest Wi-Fi, IoT devices or a lab can each get a network of their own without a new allocation.
Is a /56 enough for a small office?
It can be. Count the separate networks you need, not the devices. If that number fits within 256, a /56 covers it. If you expect to outgrow it, a /48 is the next size up, and RIPE-690 says some operators give that size to business customers.

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