What percentage of IPv6 does a /1 represent?

A /1 is exactly half of the 128-bit address space — 2¹²⁷ addresses. IPv6 has two /1 halves; global unicast (2000::/3) occupies only a tiny fraction of one of them.

What is the difference between /1 and /2?

A /1 is twice as large as /2. /1 has 2¹²⁷ addresses; /2 has 2¹²⁶. Each step down the prefix number doubles the address count.

How does /1 compare to /0?

A /0 is twice as large as /1. Moving from /1 to /0 adds one more fixed network bit, halving the host portion from 2¹²⁷ to 2¹²⁸ addresses.

How do I calculate an IPv6 /1 subnet?

Enter an IPv6 address and select prefix /1 in the calculator above — or paste full CIDR notation like 2000::/1. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2¹²⁷ addresses.

IPv6 /1 Subnet Calculator

Results update as you type

A /1 covers half of the entire 128-bit IPv6 address space — 2¹²⁷ addresses. No /1 is assigned for production use today; the IETF reserves large swaths of the address space for future purposes and special-purpose ranges.

Prefix length

/128

/1 = 2¹²⁷ addresses (≈ 1.70 × 10³⁸)

Results for ::/1

Global UnicastGlobal 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 /1

Total addresses≈ 1.70 × 10³⁸ addresses

Address typeGlobally routable scope

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

Neighboring /1 subnets

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

Hextet breakdown

20000000000000000000000000000000

NetworkSplit groupHost

Quick facts for IPv6 /1

Prefix mask: 8000::
Total addresses: 2¹²⁷
Approx. count: 1.70 × 10³⁸
/64 subnets: 2⁶³

IPv6 /1 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

mixhosthosthosthosthosthosthost

Network bits (1)Split hextetHost bits (127)

Network bits: 1
Host bits: 127
Prefix mask: 8000::
Total addresses: 2¹²⁷
Approx. count: 1.70 × 10³⁸
/64 subnets: 2⁶³
Addresses formula: 2^127
/64 relationship: 2⁶³ × /64 subnets

Overview

Common use cases

Understanding the scale of the IPv6 address space
IETF reserved-range documentation
Top-level routing theory and certification study

Key facts

A /1 fixes 1 network bits and leaves 127 host bits — 2¹²⁷ total addresses.
In network design terms, /1 is typically a global address-space boundary.
You can subnet a /1 into about 2⁶³ /64 LANs.
At site scale, /1 equals about 2⁴⁷ /48 allocations.
At ISP scale, /1 contains about 2³¹ /32 blocks.

Design guidance

A /1 is not a size you assign to a LAN or site. Treat it as documentation of how the IPv6 address space is carved at the top of the hierarchy. When studying for certifications, focus on which well-known ranges (2000::/3, fe80::/10, fc00::/7, ff00::/8) live inside or beside this block.

Practical example

Addresses like 2000::/1 fall in reserved or special-purpose space. They illustrate how a /1 boundary groups addresses for routing policy, not how you would number a home LAN.

Related RFCs and standards

RFC 4291IPv6 Addressing Architecture
RFC 3849IPv6 Documentation Address Prefix (2001:db8::/32)

Prefix sizing reference

Divide /1 into…

Prefix	Subnets	Addresses each
/2	2	2¹²⁶
/3	4	2¹²⁵
/5	16	2¹²³
/9	256	2¹¹⁹

/1 fits inside…

Supernet	Addresses	/1s inside
/0	2¹²⁸	2

Notable /1 networks

2000::/3Global unicast — all public IPv6 today
2001:db8::/32Documentation prefix (RFC 3849)

Frequently asked questions

Common questions about IPv6 /1 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