How many addresses are in an IPv6 /38?

An IPv6 /38 contains 2⁹⁰ addresses. The prefix fixes the first 38 bits; the remaining 90 bits identify individual addresses in the block.

How many /64 subnets fit in a /38?

A /38 can be divided into 2²⁶ 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 /38?

A /38 contains 2¹⁰ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

When would an organization receive a /38?

A /38 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 /38 and /39?

A /38 is twice as large as /39. /38 has 2⁹⁰ addresses; /39 has 2⁸⁹. Each step down the prefix number doubles the address count.

How does /38 compare to /37?

A /37 is twice as large as /38. Moving from /38 to /37 adds one more fixed network bit, halving the host portion from 2⁹⁰ to 2⁹¹ addresses.

IPv6 /38 Subnet Calculator

Results update as you type

A /38 block fixes the first 38 bits of the address, leaving 90 host bits and 2⁹⁰ total addresses. It subdivides into about 2²⁶ /64 LAN subnets. That is also 2¹⁰ /48 site allocations. A /38 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

/38 = 2⁹⁰ addresses (≈ 1.24 × 10²⁷)

Results for 2001:db8:a800::/38

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 /38

Total addresses≈ 1.24 × 10²⁷ addresses

Address typeGlobally routable scope

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

Neighboring /38 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 /38

Prefix mask: ffff:ffff:fc00::
Total addresses: 2⁹⁰
Approx. count: 1.24 × 10²⁷
/64 subnets: 2²⁶

IPv6 /38 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetmixhosthosthosthosthost

Network bits (38)Split hextetHost bits (90)

Network bits: 38
Host bits: 90
Prefix mask: ffff:ffff:fc00::
Total addresses: 2⁹⁰
Approx. count: 1.24 × 10²⁷
/64 subnets: 2²⁶
Addresses formula: 2^90
/64 relationship: 2²⁶ × /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 /38 fixes 38 network bits and leaves 90 host bits — 2⁹⁰ total addresses.
In network design terms, /38 is typically a enterprise or multi-site allocation.
You can subnet a /38 into about 2²⁶ /64 LANs.
At site scale, /38 equals 2¹⁰ /48 allocations.

Design guidance

A /38 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::/38 might announce the entire /38 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 /38 into…

Prefix	Subnets	Addresses each
/39	2	2⁸⁹
/40	4	2⁸⁸
/42	16	2⁸⁶
/46	256	2⁸²

/38 fits inside…

Supernet	Addresses	/38s inside
/37	2⁹¹	2
/36	2⁹²	4
/34	2⁹⁴	16
/30	2⁹⁸	256

Frequently asked questions

Common questions about IPv6 /38 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