# IPv6 /56 Prefix – 256 /64 Subnets Explained

> An IPv6 /56 prefix spans 256 /64 subnets (4,722,366,482,869,645,213,696 addresses). Examples, roles and FAQs for planning IPv6 networks.

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# IPv6 /56 Prefix ReferenceIPv6

The default homework assignment from your ISP: a /56 hands a household 256 pristine /64 subnets. If your router advertises 'IPv6 prefix delegation', this is usually the block behind it.

An IPv6 /56 prefix contains 4,722,366,482,869,645,213,696 addresses, which splits into 256 standard /64 subnets.

## /56 facts at a glance

- **Total addresses:** 4,722,366,482,869,645,213,696 (272)
- **/64 subnets inside:** 256
- **Host/interface bits:** 72
- **Example network:** 2001:db8::/56
- **First address:** 2001:db8::
- **Last address:** 2001:db8:0:ff:ffff:ffff:ffff:ffff
- **Typical role:** Home network or department allocation

## A /56 in documentation space

Using `2001:db8::/32` (RFC 3849 documentation space) aligned to its own boundary, a /56 occupies:

| Block | First address | Last address |
| --- | --- | --- |
| 2001:db8::/56 | 2001:db8:: | 2001:db8:0:ff:ffff:ffff:ffff:ffff |

## How an IPv6 /56 prefix works

A /56 prefix pins the first 56 bits of a 128-bit address, leaving 72 bits free. That free portion holds 272 = 4,722,366,482,869,645,213,696 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 8 middle bits between the prefix and the /64 boundary give 256 subnets to hand out.

## Where /56 is used

- Residential and small-business delegation via DHCPv6 prefix delegation.
- Multi-subnet homes: IoT, guests, and work on separate /64 VLANs.
- Lab racks fed downstream subnets from a single upstream handout.

A /56 contains exactly 256 /64s, enough for one subnet per VLAN with plenty to spare, no follow-up request to the ISP required.

## Frequently asked questions

**How many /64s are in a /56?**

Exactly 256. Eight delegation bits sit between /56 and /64.

**Is a /56 enough for my home network?**

Almost certainly: 256 subnets covers every VLAN a residence will ever plausibly run, with room for guests, lab gear, and future additions.

**Can I subnet a /56 further than /64?**

You can route longer prefixes manually, but hosts relying on SLAAC autoconfiguration expect a /64, so keep delegated LAN segments at exactly /64.

**Why did my ISP give me a /56 instead of a /64?**

A single /64 would lock you to one LAN. Delegating a container of subnets lets your router carve out a fresh /64 for each internal network.

## Open this prefix in the calculator

[Open 2001:db8::/56 in the IPv6 calculator](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F56)

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← IPv6 /52 prefix
IPv6 /60 prefix →

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