How many addresses are in an IPv6 /25?

An IPv6 /25 contains 2¹⁰³ addresses. The prefix fixes the first 25 bits; the remaining 103 bits identify individual addresses in the block.

How many /64 subnets fit in a /25?

A /25 can be divided into 2³⁹ 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 /25?

A /25 contains 2²³ /48 site allocations. A /48 is the classic size assigned to a business site or campus.

What is the difference between /25 and /26?

A /25 is twice as large as /26. /25 has 2¹⁰³ addresses; /26 has 2¹⁰². Each step down the prefix number doubles the address count.

How does /25 compare to /24?

A /24 is twice as large as /25. Moving from /25 to /24 adds one more fixed network bit, halving the host portion from 2¹⁰³ to 2¹⁰⁴ addresses.

Is IPv6 /25 the same as IPv4 /25?

No. IPv4 /25 has at most a few thousand hosts. IPv6 /25 still contains 2¹⁰³ addresses — an aggregation size in the IPv6 hierarchy, not a LAN subnet. IPv6 LANs are almost always /64.

IPv6 /25 Subnet Calculator

Results update as you type

A /25 block fixes the first 25 bits of the address, leaving 103 host bits and 2¹⁰³ total addresses. It subdivides into about 2³⁹ /64 LAN subnets. That is also about 2²³ /48 site allocations. At the ISP tier, a /25 holds 2⁷ /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

/25 = 2¹⁰³ addresses (≈ 1.01 × 10³¹)

Results for 2001:d80::/25

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 /25

Total addresses≈ 1.01 × 10³¹ addresses

Address typeGlobally routable scope

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

Neighboring /25 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 /25

Prefix mask: ffff:ff80::
Total addresses: 2¹⁰³
Approx. count: 1.01 × 10³¹
/64 subnets: 2³⁹

IPv6 /25 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netmixhosthosthosthosthosthost

Network bits (25)Split hextetHost bits (103)

Network bits: 25
Host bits: 103
Prefix mask: ffff:ff80::
Total addresses: 2¹⁰³
Approx. count: 1.01 × 10³¹
/64 subnets: 2³⁹
Addresses formula: 2^103
/64 relationship: 2³⁹ × /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 /25 fixes 25 network bits and leaves 103 host bits — 2¹⁰³ total addresses.
In network design terms, /25 is typically a large provider aggregation block.
You can subnet a /25 into about 2³⁹ /64 LANs.
At site scale, /25 equals about 2²³ /48 allocations.
At ISP scale, /25 contains 2⁷ /32 blocks.

Design guidance

A /25 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 /25 (or larger) upstream keeps global routing tables manageable.

Practical example

An ISP holding 2001:db8::/25 might announce the entire /25 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 /25 into…

Prefix	Subnets	Addresses each
/26	2	2¹⁰²
/27	4	2¹⁰¹
/29	16	2⁹⁹
/33	256	2⁹⁵

/25 fits inside…

Supernet	Addresses	/25s inside
/24	2¹⁰⁴	2
/23	2¹⁰⁵	4
/21	2¹⁰⁷	16
/17	2¹¹¹	256

Notable /25 networks

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

Frequently asked questions

Common questions about IPv6 /25 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