# IPv6 /40 Prefix – 16,777,216 /64 Subnets Explained

> An IPv6 /40 prefix spans 16,777,216 /64 subnets (309,485,009,821,345,068,724,781,056 addresses). Examples, roles and FAQs for planning IPv6 networks.

- Page URL: https://itinsighthub.com/ipv6/40/
- Markdown variant of this page: append `?format=md` to any URL on this site or send `Accept: text/markdown`.
- Full site index for AI assistants: https://itinsighthub.com/llms.txt
- Full site export: https://itinsighthub.com/llms-full.txt

# IPv6 /40 Prefix ReferenceIPv6

Wholesale arithmetic for hosting providers: a /40 is 256 classic site allocations deep, wrapping more than a quarter-billion /64 subnets behind one prefix. Secure one of these and you can put a /48 behind every customer cage you operate.

An IPv6 /40 prefix contains 309,485,009,821,345,068,724,781,056 addresses, which splits into 16,777,216 standard /64 subnets.

## /40 facts at a glance

- **Total addresses:** 309,485,009,821,345,068,724,781,056 (288)
- **/64 subnets inside:** 16,777,216
- **Host/interface bits:** 88
- **Example network:** 2001:db8::/40
- **First address:** 2001:db8::
- **Last address:** 2001:db8:ff:ffff:ffff:ffff:ffff:ffff
- **Typical role:** Growing ISP or multi-site enterprise

## A /40 in documentation space

Using `2001:db8::/32` (RFC 3849 documentation space) aligned to its own boundary, a /40 occupies:

| Block | First address | Last address |
| --- | --- | --- |
| 2001:db8::/40 | 2001:db8:: | 2001:db8:ff:ffff:ffff:ffff:ffff:ffff |

## How an IPv6 /40 prefix works

A /40 prefix pins the first 40 bits of a 128-bit address, leaving 88 bits free. That free portion holds 288 = 309,485,009,821,345,068,724,781,056 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 24 middle bits between the prefix and the /64 boundary give 16,777,216 subnets to hand out.

## Where /40 is used

- Hosting providers stamping out one /48 per customer or per rack.
- Large enterprises giving every business unit its own site-sized slice.
- Cloud regions pooling address space ahead of finer delegation.

A /40 descends cleanly into sixteen /44s or 256 /48s, keeping every boundary nibble-aligned for readable reverse DNS zones.

At 16,777,216 /64s, exhaustion is never the operational concern; aggregation discipline is.

## Frequently asked questions

**How many /48s are in a /40?**

Exactly 256. Eight bits separate the prefixes, which is why the /40 is beloved wherever customers are billed by the site block.

**How many /64 subnets does a /40 contain?**

16,777,216 — twenty-four bits' worth of standard subnets.

**How many addresses does an IPv6 /40 have?**

309,485,009,821,345,068,724,781,056, which works out to 2^88.

**Is a /40 a realistic enterprise assignment?**

For very large organizations, yes; more often it functions as the provider-side container from which enterprise /44s, /48s, and /56s are carved.

## Open this prefix in the calculator

[Open 2001:db8::/40 in the IPv6 calculator](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F40)

### Related IPv6 prefixes

[/29](https://itinsighthub.com/ipv6/29/)[/32](https://itinsighthub.com/ipv6/32/)[/34](https://itinsighthub.com/ipv6/34/)[/36](https://itinsighthub.com/ipv6/36/)[/42](https://itinsighthub.com/ipv6/42/)[/44](https://itinsighthub.com/ipv6/44/)[/47](https://itinsighthub.com/ipv6/47/)[/48](https://itinsighthub.com/ipv6/48/)[/52](https://itinsighthub.com/ipv6/52/)[/56](https://itinsighthub.com/ipv6/56/)[/60](https://itinsighthub.com/ipv6/60/)[/64](https://itinsighthub.com/ipv6/64/)[/127](https://itinsighthub.com/ipv6/127/)[/128](https://itinsighthub.com/ipv6/128/)

← IPv6 /36 prefix
IPv6 /42 prefix →

---

© 2026 ITInsightHub · [About](https://itinsighthub.com/about/) · [Contact](https://itinsighthub.com/contact/) · [Privacy](https://itinsighthub.com/privacy/)