How many addresses are in an IPv6 /18?

An IPv6 /18 contains 2¹¹⁰ addresses. The prefix fixes the first 18 bits; the remaining 110 bits identify individual addresses in the block.

How many /64 subnets fit in a /18?

A /18 can be divided into 2⁴⁶ 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 /18?

A /18 contains 2³⁰ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

What is the difference between /18 and /19?

A /18 is twice as large as /19. /18 has 2¹¹⁰ addresses; /19 has 2¹⁰⁹. Each step down the prefix number doubles the address count.

How does /18 compare to /17?

A /17 is twice as large as /18. Moving from /18 to /17 adds one more fixed network bit, halving the host portion from 2¹¹⁰ to 2¹¹¹ addresses.

How do I calculate an IPv6 /18 subnet?

Enter an IPv6 address and select prefix /18 in the calculator above — or paste full CIDR notation like 2001:db8::/18. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2¹¹⁰ addresses.

IPv6 /18 Subnet Calculator

Results update as you type

A /18 block fixes the first 18 bits of the address, leaving 110 host bits and 2¹¹⁰ total addresses. It subdivides into about 2⁴⁶ /64 LAN subnets. That is also about 2³⁰ /48 site allocations. At the ISP tier, a /18 holds 2¹⁴ /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

/18 = 2¹¹⁰ addresses (≈ 1.30 × 10³³)

Results for 2001::/18

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 /18

Total addresses≈ 1.30 × 10³³ addresses

Address typeGlobally routable scope

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

Neighboring /18 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 /18

Prefix mask: ffff:c000::
Total addresses: 2¹¹⁰
Approx. count: 1.30 × 10³³
/64 subnets: 2⁴⁶

IPv6 /18 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netmixhosthosthosthosthosthost

Network bits (18)Split hextetHost bits (110)

Network bits: 18
Host bits: 110
Prefix mask: ffff:c000::
Total addresses: 2¹¹⁰
Approx. count: 1.30 × 10³³
/64 subnets: 2⁴⁶
Addresses formula: 2^110
/64 relationship: 2⁴⁶ × /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 /18 fixes 18 network bits and leaves 110 host bits — 2¹¹⁰ total addresses.
In network design terms, /18 is typically a RIR-level aggregation block.
You can subnet a /18 into about 2⁴⁶ /64 LANs.
At site scale, /18 equals about 2³⁰ /48 allocations.
At ISP scale, /18 contains 2¹⁴ /32 blocks.

Design guidance

A /18 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 /18 (or larger) upstream keeps global routing tables manageable.

Practical example

An ISP holding 2001:db8::/18 might announce the entire /18 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 /18 into…

Prefix	Subnets	Addresses each
/19	2	2¹⁰⁹
/20	4	2¹⁰⁸
/22	16	2¹⁰⁶
/26	256	2¹⁰²

/18 fits inside…

Supernet	Addresses	/18s inside
/17	2¹¹¹	2
/16	2¹¹²	4
/14	2¹¹⁴	16
/10	2¹¹⁸	256

Notable /18 networks

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

Frequently asked questions

Common questions about IPv6 /18 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