Network Engineering Utility

IPv6 Subnet Calculator & Address Expander

Expand compressed IPv6 addresses, compute CIDR prefix allocations, inspect network boundaries, and generate reverse DNS ip6.arpa PTR records.

IPv6 Address & Prefix LengthSupports CIDR (/64, /48, /32) & :: notation

Presets:

Prefix Length: /64

Canonical Compressed (RFC 5952)Standard

2001:db8:abcd:12::1

Fully Expanded Address (8 Hexadectets)128-bit Full

2001:0db8:abcd:0012:0000:0000:0000:0001

Subnet Allocation & Boundary Diagnostics

Network Route Prefix

2001:db8:abcd:12::/64

Last Address in Prefix

2001:db8:abcd:12:ffff:ffff:ffff:ffff

Address Scope

Documentation

Total Available Addresses

18,446,744,073,709,551,616

SLAAC /64 Subnets

1 (/64 subnets)

Host Bits / Network Bits

64 net / 64 host

Reverse DNS PTR Record (ip6.arpa Format):

1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.1.0.0.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa

128-bit Binary Stream:

0010000000000001 0000110110111000 1010101111001101 0000000000010010 0000000000000000 0000000000000000 0000000000000000 0000000000000001

IPv6 Addressing & Subnetting Prefix Cheat Sheet1-Page PDF

Download our printable vector guide covering Global Unicast, Unique Local (ULA), Link-Local, Multicast, standard prefix allocations (/32, /48, /56, /64), and SLAAC capacities.

Download PDF

Understanding IPv6 Addressing & Subnet Structure

IPv6 addresses are 128 bits long, written as eight 16-bit blocks (hexadectets) separated by colons, represented in hexadecimal (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334). Unlike IPv4 which ran out of addresses, IPv6 provides 3.4 × 10^38 unique addresses—enough for every device on Earth to have billions of publicly routable addresses.

Global Routing Prefix (/48)

Assigned by Regional Internet Registries (RIRs) and ISPs to an enterprise site or organization.

Subnet ID (Bits 49–64)

A 16-bit field controlled by local network administrators to define up to 65,536 separate internal subnets.

Interface ID (Bits 65–128)

A 64-bit host identifier autoconfigured via SLAAC (EUI-64) or assigned via DHCPv6 / static allocation.

Standard IPv6 Prefix Allocation Matrix

Prefix Length	Designation	Number of /64 Subnets	Typical Target
/32	ISP Block	4,294,967,296	Internet Service Providers
/48	Enterprise Site	65,536	Datacenters, Corporate HQ
/56	Residential Customer	256	Home Broadband & Small Office
/64	Single LAN Subnet	1	VLAN, Wi-Fi SSID, SLAAC
/128	Single Host / Loopback	N/A (1 address)	Router loopback, VPN endpoint

Frequently Asked Questions (FAQ)

IPv4's 32-bit address space provides approximately 4.29 billion addresses, which has been completely exhausted globally. IPv6 uses a 128-bit address space, providing 3.4 x 10^38 unique addresses—enough to assign undecillions of addresses to every square meter of the Earth's surface and eliminating the need for NAT (Network Address Translation).

Diagnostic Platform