How many addresses are in an IPv6 /43?

An IPv6 /43 contains 2⁸⁵ addresses. The prefix fixes the first 43 bits; the remaining 85 bits identify individual addresses in the block.

How many /64 subnets fit in a /43?

A /43 can be divided into 2²¹ 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 /43?

A /43 contains 2⁵ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

When would an organization receive a /43?

A /43 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 /43 and /44?

A /43 is twice as large as /44. /43 has 2⁸⁵ addresses; /44 has 2⁸⁴. Each step down the prefix number doubles the address count.

How does /43 compare to /42?

A /42 is twice as large as /43. Moving from /43 to /42 adds one more fixed network bit, halving the host portion from 2⁸⁵ to 2⁸⁶ addresses.

IPv6 /43 Subnet Calculator

Results update as you type

A /43 block fixes the first 43 bits of the address, leaving 85 host bits and 2⁸⁵ total addresses. It subdivides into about 2²¹ /64 LAN subnets. That is also 2⁵ /48 site allocations. A /43 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

/43 = 2⁸⁵ addresses (≈ 3.87 × 10²⁵)

Results for 2001:db8:abc0::/43

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 /43

Total addresses≈ 3.87 × 10²⁵ addresses

Address typeGlobally routable scope

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

Neighboring /43 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 /43

Prefix mask: ffff:ffff:ffe0::
Total addresses: 2⁸⁵
Approx. count: 3.87 × 10²⁵
/64 subnets: 2²¹

IPv6 /43 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetmixhosthosthosthosthost

Network bits (43)Split hextetHost bits (85)

Network bits: 43
Host bits: 85
Prefix mask: ffff:ffff:ffe0::
Total addresses: 2⁸⁵
Approx. count: 3.87 × 10²⁵
/64 subnets: 2²¹
Addresses formula: 2^85
/64 relationship: 2²¹ × /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 /43 fixes 43 network bits and leaves 85 host bits — 2⁸⁵ total addresses.
In network design terms, /43 is typically a enterprise or multi-site allocation.
You can subnet a /43 into about 2²¹ /64 LANs.
At site scale, /43 equals 2⁵ /48 allocations.

Design guidance

A /43 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::/43 might announce the entire /43 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 /43 into…

Prefix	Subnets	Addresses each
/44	2	2⁸⁴
/45	4	2⁸³
/47	16	2⁸¹
/51	256	2⁷⁷

/43 fits inside…

Supernet	Addresses	/43s inside
/42	2⁸⁶	2
/41	2⁸⁷	4
/39	2⁸⁹	16
/35	2⁹³	256

Frequently asked questions

Common questions about IPv6 /43 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