IPv6 Subnet Calculator

Frequently Asked Questions

What is the standard site allocation size in IPv6?

RFC 6177 recommends end sites receive a /48 (65,536 /64 subnets). Residential subscribers typically get a /56 (256 /64s) or /60 (16 /64s). Each LAN segment then gets one /64.

Why does IPv6 always use /64 for LAN segments?

SLAAC generates interface identifiers requiring exactly 64 bits of host space. NDP features and privacy extensions also assume /64. Smaller prefixes break auto-configuration.

How do I read the :: shorthand?

The double colon represents consecutive all-zero groups. Count existing groups, subtract from 8, and insert that many zero groups. 2001:db8::1 expands to 2001:0db8:0000:0000:0000:0000:0000:0001.

Is IPv6 subnetting the same as IPv4 subnetting?

Conceptually similar but different in scale and convention. IPv6 uses 128-bit hex math, has no broadcast address, no classful hierarchy, and a near-universal /64 LAN convention.

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Estimates for informational purposes only.

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.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How It 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 calculator 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 using JavaScript BigInt arithmetic, 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.

Use Cases

Network engineers and ISP operators use this calculator to:

Split an ISP-assigned /32 or /48 into site allocations (/48 or /56) for customers.

Determine how many /64 networks fit inside a larger allocation for VLAN planning.

Verify that a prefix expansion does not overlap with other existing allocations in a routing table.

Plan data center pod addressing where each rack or server gets a /64 for SLAAC.

Generate documentation for IP address management (IPAM) tools by confirming network and last-address boundaries.

Tips

Always assign /64 to LAN segments - SLAAC (Stateless Address Autoconfiguration) requires a /64 host portion; using a longer prefix breaks many IPv6 features including neighbor discovery.

Allocate generously on nibble boundaries - IPv6 addresses are effectively unlimited; carving on /48, /52, /56, /60, /64 boundaries keeps prefixes legible in routing tables and firewall rules.

Use 2001:db8::/32 for documentation and testing - this reserved range is explicitly for examples and never routes on the public internet, preventing lab mistakes from polluting real routing tables.

There is no broadcast address in IPv6 - the all-ones host address is a regular unicast address, not reserved; do not subtract 2 from usable count as you would for IPv4.

Avoid subnetting below /64 for most use cases - /127 is acceptable for point-to-point links (RFC 6164), but /128 is a host route, not a subnet.

FAQ

What is the standard site allocation size in IPv6?

RFC 6177 recommends end sites receive a /48, giving them 65,536 /64 subnets - more than enough for any organization. Single residential subscribers typically receive a /56 (256 /64 subnets) or /60 (16 /64 subnets). Each LAN segment then gets one /64.

Why does IPv6 always use /64 for LAN segments?

SLAAC generates interface identifiers from the EUI-64 of the MAC address, which requires exactly 64 bits of host space. Many NDP (Neighbor Discovery Protocol) features and privacy extensions also assume a /64. Using smaller prefixes technically works for static addressing but breaks auto-configuration.

How do I read the :: shorthand?

The double colon represents one or more consecutive groups of all zeros. It can appear only once in an address. To expand it, count the existing groups, subtract from 8, and insert that many all-zero groups at the :: position. For example, 2001:db8::1 has 3 explicit groups, so :: replaces 4 zero groups: 2001:0db8:0000:0000:0000:0000:0000:0001.

Is IPv6 subnetting the same as IPv4 subnetting?

The concept is the same - a prefix fixes network bits and the host portion can be further subdivided - but the scale and conventions differ dramatically. IPv6 uses 128-bit hex math, has no broadcast address, no classful hierarchy, and a near-universal /64 LAN convention. This calculator handles IPv6-specific 128-bit arithmetic correctly.

IPv6 Subnet Calculator

Frequently Asked Questions

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