# IPv6 /47 Prefix – 131,072 /64 Subnets Explained

> An IPv6 /47 prefix spans 131,072 /64 subnets (2,417,851,639,229,258,349,412,352 addresses). Examples, roles and FAQs for planning IPv6 networks.

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# IPv6 /47 Prefix ReferenceIPv6

The pair-broker of IPv6: a /47 is precisely two adjacent /48s, and its main claim to fame is letting you announce two site blocks as one route. Beyond that trick it is an odd duck — 131,072 /64 subnets wearing a prefix found on no standard catalog page.

An IPv6 /47 prefix contains 2,417,851,639,229,258,349,412,352 addresses, which splits into 131,072 standard /64 subnets.

## /47 facts at a glance

- **Total addresses:** 2,417,851,639,229,258,349,412,352 (281)
- **/64 subnets inside:** 131,072
- **Host/interface bits:** 81
- **Example network:** 2001:db8::/47
- **First address:** 2001:db8::
- **Last address:** 2001:db8:1:ffff:ffff:ffff:ffff:ffff
- **Typical role:** Unusual split of a /46 or /48 region

## A /47 in documentation space

Using `2001:db8::/32` (RFC 3849 documentation space) aligned to its own boundary, a /47 occupies:

| Block | First address | Last address |
| --- | --- | --- |
| 2001:db8::/47 | 2001:db8:: | 2001:db8:1:ffff:ffff:ffff:ffff:ffff |

## How an IPv6 /47 prefix works

A /47 prefix pins the first 47 bits of a 128-bit address, leaving 81 bits free. That free portion holds 281 = 2,417,851,639,229,258,349,412,352 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 17 middle bits between the prefix and the /64 boundary give 131,072 subnets to hand out.

## Where /47 is used

- Aggregating two neighboring /48s into a single upstream announcement.
- Halving a /46 during top-down subdivision of a larger grant.
- Edge cases in addressing plans where two site blocks share one fate.

Arithmetic first, romance second: two /48s, four /49s, eight /50s. If your design leans on /47s, double-check that the halves really belong together.

Because /47 falls off the nibble grid, expect it to appear as internal scaffolding far more often than as an official assignment.

## Frequently asked questions

**How many /48s are in a /47?**

Two — that is the entire identity of the prefix. A single bit separates /47 from the classic site size.

**How many addresses does an IPv6 /47 have?**

2,417,851,639,229,258,349,412,352, which is 2^81.

**Should I announce a /47 instead of two /48s?**

If both blocks travel the same path, one /47 announcement trims the routing table; keep the /48s routable too so the pieces survive deaggregation.

**How many /64 subnets live in a /47?**

131,072. Seventeen bits of /64 space, split evenly across the two embedded /48s.

## Open this prefix in the calculator

[Open 2001:db8::/47 in the IPv6 calculator](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F47)

### 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/)[/40](https://itinsighthub.com/ipv6/40/)[/42](https://itinsighthub.com/ipv6/42/)[/44](https://itinsighthub.com/ipv6/44/)[/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 /44 prefix
IPv6 /48 prefix →

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