How many addresses are in an IPv6 /39?

An IPv6 /39 contains 2⁸⁹ addresses. The prefix fixes the first 39 bits; the remaining 89 bits identify individual addresses in the block.

How many /64 subnets fit in a /39?

A /39 can be divided into 2²⁵ 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 /39?

A /39 contains 2⁹ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

When would an organization receive a /39?

A /39 is assigned when an organization needs more address space than a single /48 but does not qualify for a full ISP-style /32. It is common for large enterprises with many branch offices.

What is the difference between /39 and /40?

A /39 is twice as large as /40. /39 has 2⁸⁹ addresses; /40 has 2⁸⁸. Each step down the prefix number doubles the address count.

How does /39 compare to /38?

A /38 is twice as large as /39. Moving from /39 to /38 adds one more fixed network bit, halving the host portion from 2⁸⁹ to 2⁹⁰ addresses.

IPv6 /39 Subnet Calculator

Results update as you type

A /39 block fixes the first 39 bits of the address, leaving 89 host bits and 2⁸⁹ total addresses. It subdivides into about 2²⁵ /64 LAN subnets. That is also 2⁹ /48 site allocations. A /39 sits between the standard ISP /32 and the site /48 — useful when an organization needs more than one /48 but less than a full /32.

Prefix length

/128

/39 = 2⁸⁹ addresses (≈ 6.19 × 10²⁶)

Results for 2001:db8:aa00::/39

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 /39

Total addresses≈ 6.19 × 10²⁶ addresses

Address typeGlobally routable scope

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

Neighboring /39 subnets

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

Hextet breakdown

20010db8abcd00000000000000000000

NetworkSplit groupHost

Quick facts for IPv6 /39

Prefix mask: ffff:ffff:fe00::
Total addresses: 2⁸⁹
Approx. count: 6.19 × 10²⁶
/64 subnets: 2²⁵

IPv6 /39 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetmixhosthosthosthosthost

Network bits (39)Split hextetHost bits (89)

Network bits: 39
Host bits: 89
Prefix mask: ffff:ffff:fe00::
Total addresses: 2⁸⁹
Approx. count: 6.19 × 10²⁶
/64 subnets: 2²⁵
Addresses formula: 2^89
/64 relationship: 2²⁵ × /64 subnets

Overview

Common use cases

Enterprise or ISP allocations above site size
Multi-site organizations needing several /48s
Address renumbering and growth planning

Key facts

A /39 fixes 39 network bits and leaves 89 host bits — 2⁸⁹ total addresses.
In network design terms, /39 is typically a enterprise or multi-site allocation.
You can subnet a /39 into about 2²⁵ /64 LANs.
At site scale, /39 equals 2⁹ /48 allocations.

Design guidance

A /39 suits organizations that have outgrown a single /48 but do not need a full /32 ISP allocation. Plan your addressing scheme before delegating: assign one /48 (or smaller) per major site, then subnet each site into /64 LANs. Document your nibble boundaries so future growth does not force renumbering.

Practical example

An ISP holding 2001:db8:abcd::/39 might announce the entire /39 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

Prefix sizing reference

Divide /39 into…

Prefix	Subnets	Addresses each
/40	2	2⁸⁸
/41	4	2⁸⁷
/43	16	2⁸⁵
/47	256	2⁸¹

/39 fits inside…

Supernet	Addresses	/39s inside
/38	2⁹⁰	2
/37	2⁹¹	4
/35	2⁹³	16
/31	2⁹⁷	256

Frequently asked questions

Common questions about IPv6 /39 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