How many addresses are in an IPv6 /31?

An IPv6 /31 contains 2⁹⁷ addresses. The prefix fixes the first 31 bits; the remaining 97 bits identify individual addresses in the block.

How many /64 subnets fit in a /31?

A /31 can be divided into 2³³ 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 /31?

A /31 contains 2¹⁷ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

What is the difference between /31 and /32?

A /31 is twice as large as /32. /31 has 2⁹⁷ addresses; /32 has 2⁹⁶. Each step down the prefix number doubles the address count.

How does /31 compare to /30?

A /30 is twice as large as /31. Moving from /31 to /30 adds one more fixed network bit, halving the host portion from 2⁹⁷ to 2⁹⁸ addresses.

How do I calculate an IPv6 /31 subnet?

Enter an IPv6 address and select prefix /31 in the calculator above — or paste full CIDR notation like 2001:db8::/31. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2⁹⁷ addresses.

IPv6 /31 Subnet Calculator

Results update as you type

A /31 block fixes the first 31 bits of the address, leaving 97 host bits and 2⁹⁷ total addresses. It subdivides into about 2³³ /64 LAN subnets. That is also 2¹⁷ /48 site allocations. At the ISP tier, a /31 holds 2¹ /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

/31 = 2⁹⁷ addresses (≈ 1.58 × 10²⁹)

Results for 2001:db8::/31

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 /31

Total addresses≈ 1.58 × 10²⁹ addresses

Address typeGlobally routable scope

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

Neighboring /31 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 /31

Prefix mask: ffff:fffe::
Total addresses: 2⁹⁷
Approx. count: 1.58 × 10²⁹
/64 subnets: 2³³

IPv6 /31 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netmixhosthosthosthosthosthost

Network bits (31)Split hextetHost bits (97)

Network bits: 31
Host bits: 97
Prefix mask: ffff:fffe::
Total addresses: 2⁹⁷
Approx. count: 1.58 × 10²⁹
/64 subnets: 2³³
Addresses formula: 2^97
/64 relationship: 2³³ × /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 /31 fixes 31 network bits and leaves 97 host bits — 2⁹⁷ total addresses.
In network design terms, /31 is typically a large provider aggregation block.
You can subnet a /31 into about 2³³ /64 LANs.
At site scale, /31 equals 2¹⁷ /48 allocations.
At ISP scale, /31 contains 2¹ /32 blocks.

Design guidance

A /31 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 /31 (or larger) upstream keeps global routing tables manageable.

Practical example

An ISP holding 2001:db8::/31 might announce the entire /31 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 /31 into…

Prefix	Subnets	Addresses each
/32	2	2⁹⁶
/33	4	2⁹⁵
/35	16	2⁹³
/39	256	2⁸⁹

/31 fits inside…

Supernet	Addresses	/31s inside
/30	2⁹⁸	2
/29	2⁹⁹	4
/27	2¹⁰¹	16
/23	2¹⁰⁵	256

Notable /31 networks

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

Frequently asked questions

Common questions about IPv6 /31 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