How many addresses are in an IPv6 /21?

An IPv6 /21 contains 2¹⁰⁷ addresses. The prefix fixes the first 21 bits; the remaining 107 bits identify individual addresses in the block.

How many /64 subnets fit in a /21?

A /21 can be divided into 2⁴³ 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 /21?

A /21 contains 2²⁷ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

What is the difference between /21 and /22?

A /21 is twice as large as /22. /21 has 2¹⁰⁷ addresses; /22 has 2¹⁰⁶. Each step down the prefix number doubles the address count.

How does /21 compare to /20?

A /20 is twice as large as /21. Moving from /21 to /20 adds one more fixed network bit, halving the host portion from 2¹⁰⁷ to 2¹⁰⁸ addresses.

How do I calculate an IPv6 /21 subnet?

Enter an IPv6 address and select prefix /21 in the calculator above — or paste full CIDR notation like 2001:db8::/21. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2¹⁰⁷ addresses.

IPv6 /21 Subnet Calculator

Results update as you type

A /21 block fixes the first 21 bits of the address, leaving 107 host bits and 2¹⁰⁷ total addresses. It subdivides into about 2⁴³ /64 LAN subnets. That is also about 2²⁷ /48 site allocations. At the ISP tier, a /21 holds 2¹¹ /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

/21 = 2¹⁰⁷ addresses (≈ 1.62 × 10³²)

Results for 2001:800::/21

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 /21

Total addresses≈ 1.62 × 10³² addresses

Address typeGlobally routable scope

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

Neighboring /21 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 /21

Prefix mask: ffff:f800::
Total addresses: 2¹⁰⁷
Approx. count: 1.62 × 10³²
/64 subnets: 2⁴³

IPv6 /21 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netmixhosthosthosthosthosthost

Network bits (21)Split hextetHost bits (107)

Network bits: 21
Host bits: 107
Prefix mask: ffff:f800::
Total addresses: 2¹⁰⁷
Approx. count: 1.62 × 10³²
/64 subnets: 2⁴³
Addresses formula: 2^107
/64 relationship: 2⁴³ × /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 /21 fixes 21 network bits and leaves 107 host bits — 2¹⁰⁷ total addresses.
In network design terms, /21 is typically a RIR-level aggregation block.
You can subnet a /21 into about 2⁴³ /64 LANs.
At site scale, /21 equals about 2²⁷ /48 allocations.
At ISP scale, /21 contains 2¹¹ /32 blocks.

Design guidance

A /21 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 /21 (or larger) upstream keeps global routing tables manageable.

Practical example

An ISP holding 2001:db8::/21 might announce the entire /21 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 /21 into…

Prefix	Subnets	Addresses each
/22	2	2¹⁰⁶
/23	4	2¹⁰⁵
/25	16	2¹⁰³
/29	256	2⁹⁹

/21 fits inside…

Supernet	Addresses	/21s inside
/20	2¹⁰⁸	2
/19	2¹⁰⁹	4
/17	2¹¹¹	16
/13	2¹¹⁵	256

Notable /21 networks

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

Frequently asked questions

Common questions about IPv6 /21 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