# IPv6 /42 Prefix – 4,194,304 /64 Subnets Explained

> An IPv6 /42 prefix spans 4,194,304 /64 subnets (77,371,252,455,336,267,181,195,264 addresses). Examples, roles and FAQs for planning IPv6 networks.

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# IPv6 /42 Prefix ReferenceIPv6

Nobody orders a /42 by name, yet it keeps showing up: quarter a /40 and four of these fall out, each carrying 4,194,304 /64 subnets. Think of it as the honest middle step between a provider's /40 and a tenant's /44.

An IPv6 /42 prefix contains 77,371,252,455,336,267,181,195,264 addresses, which splits into 4,194,304 standard /64 subnets.

## /42 facts at a glance

- **Total addresses:** 77,371,252,455,336,267,181,195,264 (286)
- **/64 subnets inside:** 4,194,304
- **Host/interface bits:** 86
- **Example network:** 2001:db8::/42
- **First address:** 2001:db8::
- **Last address:** 2001:db8:3f:ffff:ffff:ffff:ffff:ffff
- **Typical role:** Aggregation prefix between allocations

## A /42 in documentation space

Using `2001:db8::/32` (RFC 3849 documentation space) aligned to its own boundary, a /42 occupies:

| Block | First address | Last address |
| --- | --- | --- |
| 2001:db8::/42 | 2001:db8:: | 2001:db8:3f:ffff:ffff:ffff:ffff:ffff |

## How an IPv6 /42 prefix works

A /42 prefix pins the first 42 bits of a 128-bit address, leaving 86 bits free. That free portion holds 286 = 77,371,252,455,336,267,181,195,264 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 22 middle bits between the prefix and the /64 boundary give 4,194,304 subnets to hand out.

## Where /42 is used

- Quartering a /40 into four chunks, each ready to split further into /44s.
- Intermediate staging during top-down subdivision of larger aggregates.
- Lab and training environments practicing hierarchical delegation math.

The counts stay friendly: one /42 equals four /44s or sixty-four /48s.

Because /42 misses the nibble grid, it almost always plays the role of internal scaffolding rather than a formally issued grant.

## Frequently asked questions

**How many /44s are in a /42?**

Four. Two bits of difference, so halving twice takes you from /42 down to /44.

**How many /48s fit in a /42?**

Sixty-four — six bits' worth of classic site allocations.

**How many addresses does an IPv6 /42 have?**

77,371,252,455,336,267,181,195,264, which is 2^86.

## Open this prefix in the calculator

[Open 2001:db8::/42 in the IPv6 calculator](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F42)

### 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/)[/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 /40 prefix
IPv6 /44 prefix →

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