How big is an IPv6 /16?

A /16 holds 2¹¹² addresses. It can be split into 65,536 /32 allocations, and each /32 can be split further into millions of /48 sites — so a single /16 represents an enormous amount of address space.

What is the difference between /16 and /17?

A /16 is twice as large as /17. /16 has 2¹¹² addresses; /17 has 2¹¹¹. Each step down the prefix number doubles the address count.

How does /16 compare to /15?

A /15 is twice as large as /16. Moving from /16 to /15 adds one more fixed network bit, halving the host portion from 2¹¹² to 2¹¹³ addresses.

How do I calculate an IPv6 /16 subnet?

Enter an IPv6 address and select prefix /16 in the calculator above — or paste full CIDR notation like 2001:db8::/16. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2¹¹² addresses.

IPv6 /16 Subnet Calculator

Results update as you type

A /16 is a very large allocation, often held by major ISPs or used to group an RIR’s assignments. It contains 2¹¹² addresses and can be divided into 65,536 /32s or over 2⁴⁸ standard /64 LANs.

Prefix length

/128

/16 = 2¹¹² addresses (≈ 5.19 × 10³³)

Results for 2001::/16

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 /16

Total addresses≈ 5.19 × 10³³ addresses

Address typeGlobally routable scope

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

Neighboring /16 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 /16

Prefix mask: ffff::
Total addresses: 2¹¹²
Approx. count: 5.19 × 10³³
/64 subnets: 2⁴⁸

IPv6 /16 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

nethosthosthosthosthosthosthost

Network bits (16)Split hextetHost bits (112)

Network bits: 16
Host bits: 112
Prefix mask: ffff::
Total addresses: 2¹¹²
Approx. count: 5.19 × 10³³
/64 subnets: 2⁴⁸
Addresses formula: 2^112
/64 relationship: 2⁴⁸ × /64 subnets

Overview

Common use cases

Very large ISP backbone allocations
Address-space aggregation for routing

Key facts

A /16 fixes 16 network bits and leaves 112 host bits — 2¹¹² total addresses.
In network design terms, /16 is typically a RIR-level aggregation block.
You can subnet a /16 into about 2⁴⁸ /64 LANs.
At site scale, /16 equals about 2³² /48 allocations.
At ISP scale, /16 contains 2¹⁶ /32 blocks.

Design guidance

A /16 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 /16 (or larger) upstream keeps global routing tables manageable.

Practical example

An ISP holding 2001:db8::/16 might announce the entire /16 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 /16 into…

Prefix	Subnets	Addresses each
/17	2	2¹¹¹
/18	4	2¹¹⁰
/20	16	2¹⁰⁸
/24	256	2¹⁰⁴

/16 fits inside…

Supernet	Addresses	/16s inside
/15	2¹¹³	2
/14	2¹¹⁴	4
/12	2¹¹⁶	16
/8	2¹²⁰	256

Notable /16 networks

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

Frequently asked questions

Common questions about IPv6 /16 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