IPv6 Subnet Calculator

Frequently Asked Questions

How does IPv6 subnetting differ from IPv4?

IPv6 uses 128-bit addresses in hex. Subnets are huge: a /64 holds 2^64 addresses, and going from /48 to /64 yields 2^(64−48) = 65,536 subnets. The tool computes these from your prefixes.

Why is /64 the standard subnet size?

SLAAC (stateless address autoconfiguration) requires a 64-bit interface identifier, so /64 is the conventional LAN size regardless of host count.

What is a typical allocation hierarchy?

An ISP may assign a site a /48, which provides 65,536 /64 subnets - plenty for any organization. The calculator shows subnet counts and addresses per prefix.

How do I read :: in an address?

The double colon represents one run of all-zero groups, expanded so the address totals eight hextets. The tool expands it when computing ranges.

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Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

An IPv6 address is 128 bits, written as eight 16-bit hex groups. A prefix like 2001:db8::/32 fixes the first 32 bits as the network; the remaining 96 bits are yours to subdivide. Carving a longer target prefix out of a shorter one creates 2^{(new − old)} subnets, each holding 2^{(128 − new)} addresses. The tool expands the :: shorthand (which stands in for one or more all-zero groups) into the full eight-group form, converts it to a 128-bit integer, applies the bitwise prefix mask to find the network base, then computes the first and last address of the block. Because subnet boundaries fall on nibble (4-bit) lines in hex, /48, /52, /56, /60 and /64 are the practical, human-readable cut points.

Worked Example

Allocate /64s from a /48: 2001:db8:abcd::/48 split into /64s. Subnet bits = 64 − 48 = 16, so you get 2¹⁶ = 65,536 separate /64 networks. Each /64 still contains 2^(128−64) = 2⁶⁴ ≈ 1.8 × 10¹⁹ addresses - more than the entire IPv4 Internet, per LAN. The first subnet is 2001:db8:abcd::/64, the next is 2001:db8:abcd:1::/64, and so on through 2001:db8:abcd:ffff::/64. This is exactly how an ISP hands a site a /48 and the site assigns one /64 per VLAN.

Real-World Allocation Practice

RFC-recommended boundaries: an end site typically receives a /48 (65,536 /64s), a single subscriber often a /56 (256 /64s), and one LAN segment always a /64 - SLAAC and many features assume a /64 host portion. Don't "conserve" IPv6 the way you ration IPv4: addresses are effectively unlimited, so allocate generously on nibble boundaries to keep prefixes legible and route aggregation clean. Documentation and labs should use the reserved 2001:db8::/32 range so examples never collide with real routed space.

Common Mistakes

Subnetting longer than /64: a /112 or /127 works for point-to-point links but breaks SLAAC; keep host networks at /64.

Mis-expanding ::: the double colon may appear only once and represents all the missing zero groups - ::1 is not the same as 1::.

Confusing addresses with IPv4 math: there is no broadcast address and no "minus 2" for network/broadcast in IPv6.

Off-by-one on subnet count: the number of subnets is 2^(new−old), not (new − old).

How This Differs From IPv4 Subnetting

This is deliberately separate from the IPv4 Subnet / CIDR tool. IPv6 uses 128-bit hex math with no broadcast address, no usable-host subtraction, and a near-universal /64 LAN convention - concepts that simply don't map onto 32-bit dotted-decimal IPv4. The arithmetic here focuses on prefix delegation: how many subnets a longer prefix yields and how many /64s fit inside an allocation, which is the daily reality of IPv6 network design rather than host counting.

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