How many addresses are in an IPv6 /42?

An IPv6 /42 contains 2⁸⁶ addresses. The prefix fixes the first 42 bits; the remaining 86 bits identify individual addresses in the block.

How many /64 subnets fit in a /42?

A /42 can be divided into 2²² standard /64 LAN subnets. Each /64 holds 2⁶⁴ addresses, so the combined block is effectively inexhaustible for normal use.

How many /48 site allocations are in a /42?

A /42 contains 2⁶ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

When would an organization receive a /42?

A /42 is assigned when an organization needs more address space than a single /48 but does not qualify for a full ISP-style /32. It is common for large enterprises with many branch offices.

What is the difference between /42 and /43?

A /42 is twice as large as /43. /42 has 2⁸⁶ addresses; /43 has 2⁸⁵. Each step down the prefix number doubles the address count.

How does /42 compare to /41?

A /41 is twice as large as /42. Moving from /42 to /41 adds one more fixed network bit, halving the host portion from 2⁸⁶ to 2⁸⁷ addresses.

IPv6 /42 Subnet Calculator

Results update as you type

A /42 block fixes the first 42 bits of the address, leaving 86 host bits and 2⁸⁶ total addresses. It subdivides into about 2²² /64 LAN subnets. That is also 2⁶ /48 site allocations. A /42 sits between the standard ISP /32 and the site /48 — useful when an organization needs more than one /48 but less than a full /32.

Prefix length

/128

/42 = 2⁸⁶ addresses (≈ 7.74 × 10²⁵)

Results for 2001:db8:abc0::/42

Documentation (RFC 3849)Global scope

Network / prefixThe first address — identifies the subnet itself

First addressSubnet-router anycast; first address in the block

Last addressThe highest address in this block

Prefix maskEquivalent to /42

Total addresses≈ 7.74 × 10²⁵ addresses

Address typeGlobally routable scope

More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown

Neighboring /42 subnets

Expanded address
Compressed address
Network (expanded)
Last address (expanded)
Prefix mask
Total addresses (exact)
Reverse DNS (PTR)
Host bits / network bits

Hextet breakdown

20010db8abcd00000000000000000000

NetworkSplit groupHost

Quick facts for IPv6 /42

Prefix mask: ffff:ffff:ffc0::
Total addresses: 2⁸⁶
Approx. count: 7.74 × 10²⁵
/64 subnets: 2²²

IPv6 /42 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetmixhosthosthosthosthost

Network bits (42)Split hextetHost bits (86)

Network bits: 42
Host bits: 86
Prefix mask: ffff:ffff:ffc0::
Total addresses: 2⁸⁶
Approx. count: 7.74 × 10²⁵
/64 subnets: 2²²
Addresses formula: 2^86
/64 relationship: 2²² × /64 subnets

Overview

Common use cases

Enterprise or ISP allocations above site size
Multi-site organizations needing several /48s
Address renumbering and growth planning

Key facts

A /42 fixes 42 network bits and leaves 86 host bits — 2⁸⁶ total addresses.
In network design terms, /42 is typically a enterprise or multi-site allocation.
You can subnet a /42 into about 2²² /64 LANs.
At site scale, /42 equals 2⁶ /48 allocations.

Design guidance

A /42 suits organizations that have outgrown a single /48 but do not need a full /32 ISP allocation. Plan your addressing scheme before delegating: assign one /48 (or smaller) per major site, then subnet each site into /64 LANs. Document your nibble boundaries so future growth does not force renumbering.

Practical example

An ISP holding 2001:db8:abcd::/42 might announce the entire /42 to upstream providers as one BGP route, then delegate /48 blocks such as 2001:db8:0001::/48 and 2001:db8:0002::/48 to business customers. Each customer subnets their /48 into /64 LANs.

Related RFCs and standards

RFC 4291IPv6 Addressing Architecture

Prefix sizing reference

Divide /42 into…

Prefix	Subnets	Addresses each
/43	2	2⁸⁵
/44	4	2⁸⁴
/46	16	2⁸²
/50	256	2⁷⁸

/42 fits inside…

Supernet	Addresses	/42s inside
/41	2⁸⁷	2
/40	2⁸⁸	4
/38	2⁹⁰	16
/34	2⁹⁴	256

Frequently asked questions

Common questions about IPv6 /42 blocks, prefix sizes, and use cases.

Common IPv6 prefix sizes

/3 /12 /16 /20 /24 /28 /32 /36 /40 /44 /48 /52 /56 /60 /64 /80 /96 /112 /120 /124 /126 /127 /128