How many addresses are in an IPv6 /37?

An IPv6 /37 contains 2⁹¹ addresses. The prefix fixes the first 37 bits; the remaining 91 bits identify individual addresses in the block.

How many /64 subnets fit in a /37?

A /37 can be divided into 2²⁷ 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 /37?

A /37 contains 2¹¹ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

When would an organization receive a /37?

A /37 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 /37 and /38?

A /37 is twice as large as /38. /37 has 2⁹¹ addresses; /38 has 2⁹⁰. Each step down the prefix number doubles the address count.

How does /37 compare to /36?

A /36 is twice as large as /37. Moving from /37 to /36 adds one more fixed network bit, halving the host portion from 2⁹¹ to 2⁹² addresses.

IPv6 /37 Subnet Calculator

Results update as you type

A /37 block fixes the first 37 bits of the address, leaving 91 host bits and 2⁹¹ total addresses. It subdivides into about 2²⁷ /64 LAN subnets. That is also 2¹¹ /48 site allocations. A /37 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

/37 = 2⁹¹ addresses (≈ 2.48 × 10²⁷)

Results for 2001:db8:a800::/37

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 /37

Total addresses≈ 2.48 × 10²⁷ addresses

Address typeGlobally routable scope

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

Neighboring /37 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 /37

Prefix mask: ffff:ffff:f800::
Total addresses: 2⁹¹
Approx. count: 2.48 × 10²⁷
/64 subnets: 2²⁷

IPv6 /37 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetmixhosthosthosthosthost

Network bits (37)Split hextetHost bits (91)

Network bits: 37
Host bits: 91
Prefix mask: ffff:ffff:f800::
Total addresses: 2⁹¹
Approx. count: 2.48 × 10²⁷
/64 subnets: 2²⁷
Addresses formula: 2^91
/64 relationship: 2²⁷ × /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 /37 fixes 37 network bits and leaves 91 host bits — 2⁹¹ total addresses.
In network design terms, /37 is typically a enterprise or multi-site allocation.
You can subnet a /37 into about 2²⁷ /64 LANs.
At site scale, /37 equals 2¹¹ /48 allocations.

Design guidance

A /37 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::/37 might announce the entire /37 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 /37 into…

Prefix	Subnets	Addresses each
/38	2	2⁹⁰
/39	4	2⁸⁹
/41	16	2⁸⁷
/45	256	2⁸³

/37 fits inside…

Supernet	Addresses	/37s inside
/36	2⁹²	2
/35	2⁹³	4
/33	2⁹⁵	16
/29	2⁹⁹	256

Frequently asked questions

Common questions about IPv6 /37 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