IPv6 subnet guide

IPv6 CIDR notation explained

CIDR notation defines an IPv6 network prefix, its address range, and its capacity. Use this guide to read prefixes such as 2001:db8::/48 with confidence.

Prefix length

What does /48 mean in IPv6?

An IPv6 address has 128 binary bits. The number after the slash is the prefix length: it tells you how many leading bits identify the network. The remaining bits identify addresses inside that network.

For 2001:db8::/48, the first 48 bits are fixed network bits and 80 bits remain for addresses within the prefix. The network base is the first address, where every host bit is zero.

First and last address

How an IPv6 range is calculated

  1. 1Keep the prefix bits from the entered address.
  2. 2Set every remaining host bit to 0 for the first address.
  3. 3Set every host bit to 1 for the last address.

Address capacity

How many addresses are in a prefix?

An IPv6 prefix with length n contains 2^(128-n) addresses. Unlike IPv4, IPv6 subnet planning typically assigns a /64 to each LAN or broadcast domain.

/64 = 2^64 addresses = 18.44 quintillion addresses

Reference table

Common IPv6 subnet sizes

/48 80 host bits 65,536 /64 networks addresses Common site allocation
/56 72 host bits 256 /64 networks addresses Common residential delegation
/64 64 host bits 18.44 quintillion addresses Standard LAN subnet
/96 32 host bits 4.29 billion addresses IPv4-embedded address formats
/128 0 host bits 1 addresses Single IPv6 address

Reference prefixes

Special-purpose IPv6 CIDR ranges

These prefixes have a defined role in IPv6. Each link opens the calculator with the normalized prefix and its range details.

::/128 Unspecified IPv6 address The unspecified address represents the absence of an IPv6 address. It is used only as a source address during initialization and is never a routable destination. ::1/128 IPv6 loopback address The IPv6 loopback address refers back to the local device. It is the IPv6 equivalent of IPv4 localhost 127.0.0.1 and never leaves the host. 2001:db8::/32 IPv6 documentation prefix This prefix is reserved for documentation and examples. Use it in guides, diagrams, and configuration samples instead of a real routed network. fc00::/7 Unique Local Address range Unique Local Addresses are intended for private IPv6 networks. They are not globally routed on the public internet. fd00::/8 Locally assigned Unique Local Address range The fd00::/8 block is commonly used when locally generating Unique Local Addresses for internal IPv6 networks. fe80::/10 IPv6 link-local range Every IPv6-enabled interface can use a link-local address from this range. These addresses work only on the local network link. ff00::/8 IPv6 multicast range IPv6 multicast addresses identify groups of interfaces. Their scope and group ID determine where packets can be delivered.

Try a prefix

Calculate your IPv6 CIDR range

Use the calculator for any valid IPv6 CIDR prefix. It normalizes host bits and returns the precise first and last address without calling an external service.

Open CIDR calculator