IPv6 Subnet Calculator — Prefix & CIDR Tool

Range, size, /64 count, address type and reverse DNS zone of any IPv6 prefix, with a subnet planner and an address check.

IPv6 address or prefix (for example 2001:db8::/48)

Prefix length

Result

Addresses

Range –

/64 networks inside

BroadcastNone in IPv6

Field	Value

Plan subnets inside

Split into

#	Subnet	Last address

Find subnet number

Is an IPv6 address in this prefix?

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IPv6 Subnet Calculator — What the Results Tell You

IPv6 subnet calculation operates on 128-bit addresses using prefix length notation — /0 through /128 — in place of IPv4's dotted-decimal subnet mask. A /64 prefix reserves 64 bits for the network and leaves 64 bits for host addresses, yielding 2⁶⁴ addresses per subnet. IPv6 has no broadcast address — all addresses in a prefix are usable in principle. The tool outputs two representations of the address per RFC 5952, the network prefix, and the number of available /64 subnets for any shorter-than-/64 allocation.

Prefix	Typical Assignment	Total Addresses	Standard
/32	ISP allocation from RIR	2⁹⁶	RFC 6177
/48	Organization / customer site	2⁸⁰	RFC 6177
/56	Residential customer site	2⁷²	RFC 6177
/64	Single LAN subnet — SLAAC required	2⁶⁴ (~18.4 quintillion)	RFC 4291, RFC 4862
/127	Point-to-point router link	2	RFC 6164
/128	Single host — loopback, anycast	1	RFC 4291

IPv6 Address Types — Reference Table

IPv6 defines several address categories by prefix range. Knowing which range an address falls into determines how it can be routed and used.

Prefix	Type	IPv4 Equivalent	RFC
2000::/3	Global Unicast (GUA) — publicly routable	Public IPv4	RFC 4291
fc00::/7	Unique Local Address (ULA) — private use	RFC 1918 (10.x, 172.16.x, 192.168.x)	RFC 4193
fe80::/10	Link-Local — single link only, not routed	APIPA (169.254.x.x)	RFC 4291
::1/128	Loopback	127.0.0.1	RFC 4291
ff00::/8	Multicast — replaces IPv4 broadcast	224.0.0.0/4	RFC 4291
2001:db8::/32	Documentation / examples — not routable	192.0.2.0/24 (TEST-NET)	RFC 3849

How to Use the IPv6 Subnet Calculator

Type or paste an address such as 2001:db8::1, or a prefix such as 2001:db8::/48. Compressed, expanded and embedded IPv4 forms (::ffff:192.0.2.1) are accepted; results update as you type.

Set the Prefix Length

Type a prefix from /0 to /128, or pick a common size: /32 for an ISP, /48 for a site, /56 for a home or small site, /64 for one LAN, /127 for a router link, /128 for one host.

Read the Range and Size

The result shows the first and last address (compressed and expanded), the number of addresses, how many /64 networks fit inside, the address type and the ip6.arpa reverse DNS zone, each with a copy button.

Plan Subnets

Choose a smaller size, for example /48 into /64, to list the first subnets and copy them, or type a number to get subnet number N directly.

Check an Address

Paste any IPv6 address to see whether it belongs to the prefix, and which /64 it is in.

How IPv6 Subnetting Differs from IPv4

IPv6 addresses are 128 bits long (RFC 4291) — four times the width of IPv4's 32-bit address. The address space is 2¹²⁸, or approximately 3.4 × 10³⁸ addresses. This eliminates the scarcity that drove IPv4's complex subnet planning; a single /48 prefix contains 2⁸⁰ addresses — more than the entire IPv4 internet.

IPv6 has no broadcast address. Broadcast functionality is replaced by multicast (ff00::/8, RFC 4291). This eliminates the network/broadcast address reservations that reduce IPv4 usable hosts — all addresses in an IPv6 prefix are assignable.

IPv6 uses no dotted-decimal subnet mask. The equivalent is the prefix length, written in CIDR notation (e.g. /64). There is no "255.255.255.0" notation in IPv6 — only /0 through /128.

SLAAC (Stateless Address Autoconfiguration, RFC 4862) enables IPv6 hosts to self-configure an address without DHCP. SLAAC requires exactly 64 host bits — meaning the network prefix must be /64. Using longer prefixes (/65 through /127) on a LAN breaks SLAAC and is incompatible with most dual-stack deployments.

Address compression (RFC 5952) removes leading zeros from each 16-bit group and collapses the longest consecutive run of zero groups into :: — which may appear only once. This rule produces a unique canonical form for every address, eliminating notation ambiguity in routing tables, firewall rules, and logs.

Common Uses for an IPv6 Subnet Calculator

ISP address delegation planning. An ISP receives a /32 prefix from their RIR (per RFC 6177) and needs to allocate /48 blocks to enterprise customers and /56 blocks to residential customers. The calculator shows how many /48 subnets fit inside the /32 — 65,536 — and confirms the prefix boundaries before configuring the routing infrastructure.

Firewall ACL documentation. A security engineer writing an IPv6 access control list needs to confirm the canonical compressed form of a prefix before committing it to change control documentation. A typo in hexadecimal — or inconsistent use of compressed versus expanded notation — creates ambiguity in tickets and audit trails. The calculator provides the RFC 5952 canonical form and the fully expanded form for deterministic comparison.

Point-to-point router link addressing. A network engineer configuring an inter-router link assigns a /127 prefix (RFC 6164) instead of a /126, conserving address space. The calculator confirms that a /127 contains exactly 2 addresses with no broadcast — both fully usable — and displays the network prefix for the router configuration.

Frequently Asked Questions

What is an IPv6 prefix length?

An IPv6 prefix length — written after a slash, e.g. /64 — specifies how many of the 128 bits of an IPv6 address identify the network, with the remaining bits identifying the host. IPv6 uses prefix length notation exclusively; there is no dotted-decimal subnet mask equivalent as in IPv4. Prefix lengths run from /0 (the entire address space) to /128 (a single host address), per RFC 4291.

How many addresses are in a /64 IPv6 subnet?

A /64 IPv6 subnet contains 2 raised to the power of 64 addresses — approximately 18.4 quintillion (18,446,744,073,709,551,616). This is the standard allocation for a single LAN segment. Unlike IPv4, IPv6 does not subtract addresses for network or broadcast — there is no broadcast address in IPv6. All 2⁶⁴ addresses are usable in principle.

Why must a LAN subnet use /64 and not a longer prefix?

SLAAC — Stateless Address Autoconfiguration (RFC 4862) — requires exactly 64 host bits to generate an interface identifier from the device's MAC address using the Modified EUI-64 algorithm (RFC 4291). A prefix longer than /64 (e.g. /65, /80, /120) breaks SLAAC and is incompatible with many DHCPv6 implementations. For this reason, /64 is the universal standard for individual LAN subnets regardless of how few devices the segment actually contains.

What is the difference between compressed and expanded IPv6 notation?

Expanded notation writes all 128 bits as eight four-character hexadecimal groups separated by colons — e.g. 2001:0db8:0000:0000:0000:0000:0000:0001. Compressed (canonical) notation, defined in RFC 5952, removes leading zeros from each group and replaces the longest consecutive run of all-zero groups with :: — giving 2001:db8::1. Both representations are binary-identical; compression is purely cosmetic. RFC 5952 also mandates lowercase letters as the canonical form.

Do you store the IPv6 addresses entered into the calculator?

No. The IPv6 subnet calculator runs entirely in your browser using JavaScript — no input data is transmitted to a server, and no IPv6 addresses or prefix lengths are logged or stored.

Why does the /64 subnet count show 1 for prefixes longer than /64?

The available /64 subnet count is only meaningful when the entered prefix is shorter than /64 — because a prefix longer than /64 is itself smaller than a single /64 block. A /128 is a single host address; a /96 contains 2³² addresses but is a subdivision of one /64. The calculator returns "1" for any prefix of /65 or longer to indicate that the block fits within — not subdivides — a /64 subnet.

Related Guides

Guide What is IPv6?

the 128-bit addressing architecture, the transition from IPv4, and how prefix delegation works across ISPs, organizations, and individual subnets.

Guide What is an IP address?

understand the fundamentals of IP addressing that apply to both IPv4 and IPv6 networks.

Guide What is IPv4?

compare the 32-bit IPv4 subnetting model with IPv6 to understand why prefix lengths replaced dotted-decimal subnet masks.