How many addresses are in an IPv6 /23?

An IPv6 /23 contains 2¹⁰⁵ addresses. The prefix fixes the first 23 bits; the remaining 105 bits identify individual addresses in the block.

How many /64 subnets fit in a /23?

A /23 can be divided into 2⁴¹ 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 /23?

A /23 contains 2²⁵ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

What is the difference between /23 and /24?

A /23 is twice as large as /24. /23 has 2¹⁰⁵ addresses; /24 has 2¹⁰⁴. Each step down the prefix number doubles the address count.

How does /23 compare to /22?

A /22 is twice as large as /23. Moving from /23 to /22 adds one more fixed network bit, halving the host portion from 2¹⁰⁵ to 2¹⁰⁶ addresses.

How do I calculate an IPv6 /23 subnet?

Enter an IPv6 address and select prefix /23 in the calculator above — or paste full CIDR notation like 2001:db8::/23. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2¹⁰⁵ addresses.

IPv6 /23 Subnet Calculator

Results update as you type

A /23 block fixes the first 23 bits of the address, leaving 105 host bits and 2¹⁰⁵ total addresses. It subdivides into about 2⁴¹ /64 LAN subnets. That is also about 2²⁵ /48 site allocations. At the ISP tier, a /23 holds 2⁹ /32 ISP blocks. Prefixes in this range appear in BGP tables and RIR allocation policies as aggregation blocks above the standard ISP /32.

Prefix length

/128

/23 = 2¹⁰⁵ addresses (≈ 4.06 × 10³¹)

Results for 2001:c00::/23

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 /23

Total addresses≈ 4.06 × 10³¹ addresses

Address typeGlobally routable scope

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

Neighboring /23 subnets

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

Hextet breakdown

20010db8000000000000000000000000

NetworkSplit groupHost

Quick facts for IPv6 /23

Prefix mask: ffff:fe00::
Total addresses: 2¹⁰⁵
Approx. count: 4.06 × 10³¹
/64 subnets: 2⁴¹

IPv6 /23 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netmixhosthosthosthosthosthost

Network bits (23)Split hextetHost bits (105)

Network bits: 23
Host bits: 105
Prefix mask: ffff:fe00::
Total addresses: 2¹⁰⁵
Approx. count: 4.06 × 10³¹
/64 subnets: 2⁴¹
Addresses formula: 2^105
/64 relationship: 2⁴¹ × /64 subnets

Overview

Common use cases

Aggregating ISP /32 allocations in routing tables
Regional provider address planning
Large-scale subnetting study and documentation

Key facts

A /23 fixes 23 network bits and leaves 105 host bits — 2¹⁰⁵ total addresses.
In network design terms, /23 is typically a RIR-level aggregation block.
You can subnet a /23 into about 2⁴¹ /64 LANs.
At site scale, /23 equals about 2²⁵ /48 allocations.
At ISP scale, /23 contains 2⁹ /32 blocks.

Design guidance

A /23 belongs in BGP aggregation and RIR allocation planning — not on a VLAN interface. If you are subnetting for a real deployment, work downward: carve /48 or /56 site blocks first, then assign one /64 per LAN. Announcing a single aggregated /23 (or larger) upstream keeps global routing tables manageable.

Practical example

An ISP holding 2001:db8::/23 might announce the entire /23 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
RFC 3849IPv6 Documentation Address Prefix (2001:db8::/32)

Prefix sizing reference

Divide /23 into…

Prefix	Subnets	Addresses each
/24	2	2¹⁰⁴
/25	4	2¹⁰³
/27	16	2¹⁰¹
/31	256	2⁹⁷

/23 fits inside…

Supernet	Addresses	/23s inside
/22	2¹⁰⁶	2
/21	2¹⁰⁷	4
/19	2¹⁰⁹	16
/15	2¹¹³	256

Notable /23 networks

2001:db8::/32Documentation prefix (RFC 3849)

Frequently asked questions

Common questions about IPv6 /23 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