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IPv6 Subnet Calculator and Prefix Planner (/48, /56, /64)

Expand and compress IPv6 addresses per RFC 5952, calculate network prefix boundaries, /64 SLAAC subnet counts, and child prefix blocks.

Input Parameters

Try Example:
0128

Calculation Results

Prefix /48 Bit Allocation (128-bit Address Space)
Network: 48 bit (38%)Host: 80 bit (62%)
Input IPv6 Address
2001:db8:abcd:12::1
CIDR Prefix
48 bits
Compressed IPv6 Address (RFC 5952)
2001:db8:abcd:12::1
Expanded IPv6 Address
2001:0db8:abcd:0012:0000:0000:0000:0001
IPv6 Network Prefix (CIDR)
2001:db8:abcd::/48
Compressed Network Address
2001:db8:abcd::
Expanded Network Address
2001:0db8:abcd:0000:0000:0000:0000:0000
First Address in Range
2001:db8:abcd::
Last Address in Range (Compressed)
2001:db8:abcd:ffff:ffff:ffff:ffff:ffff
Last Address in Range (Expanded)
2001:0db8:abcd:ffff:ffff:ffff:ffff:ffff
Prefix Mask (Compressed)
ffff:ffff:ffff::
Prefix Mask (Expanded)
ffff:ffff:ffff:0000:0000:0000:0000:0000
Host Bits
80 bits
Total IPv6 Addresses
1.208.925.819.614.629.174.706.176
Total Addresses Power
2^80
Number of /64 Subnets (SLAAC)
65.536
Subnet /64 Power
2^16
IP Address Type
Documentation (2001:db8::/32)
Reverse DNS Zone (ip6.arpa)
d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa
Target Child Prefix
64 bits
Child Subnet Count
65.536
Child Subnet Power
2^16
Want to compare 2 design options side-by-side (e.g., Scenario A vs Scenario B)?
⚡ Enterprise Network Infrastructure Standard
Managed L2+/L3 Gigabit Switch (802.1Q VLAN & RSTP) + Cat6/FO Uplink
RFC 1918 / RFC 4632 Compliant Addressing

Isolate management, server, user, and IoT/CCTV traffic into dedicated 802.1Q VLANs.

#Child PrefixFirst AddressLast Address/64 Subnets
12001:db8:abcd::/642001:db8:abcd::2001:db8:abcd:0:ffff:ffff:ffff:ffff1
22001:db8:abcd:1::/642001:db8:abcd:1::2001:db8:abcd:1:ffff:ffff:ffff:ffff1
32001:db8:abcd:2::/642001:db8:abcd:2::2001:db8:abcd:2:ffff:ffff:ffff:ffff1
42001:db8:abcd:3::/642001:db8:abcd:3::2001:db8:abcd:3:ffff:ffff:ffff:ffff1
52001:db8:abcd:4::/642001:db8:abcd:4::2001:db8:abcd:4:ffff:ffff:ffff:ffff1
62001:db8:abcd:5::/642001:db8:abcd:5::2001:db8:abcd:5:ffff:ffff:ffff:ffff1
72001:db8:abcd:6::/642001:db8:abcd:6::2001:db8:abcd:6:ffff:ffff:ffff:ffff1
82001:db8:abcd:7::/642001:db8:abcd:7::2001:db8:abcd:7:ffff:ffff:ffff:ffff1
Auto Visual Architecture

Visual Network Topology Generator (SVG / PNG)

Network Architecture & Subnet Allocation (192.168.1.0/24)KalkuNet.com • Auto-TopologyISP / Upstream BackboneBGP / Default Route 0.0.0.RTRCore Router / L3 FirewallPrefix 192.168.1.0/24 • Mask 255SWManaged L2/L3 Distribution VLAN Trunking & Spanning Tree (RVLAN 10: Segmen 12001:db8:abcd::/64 (1 Host[Subnet Block]VLAN 20: Segmen 22001:db8:abcd:1::/64 (1 Ho[Subnet Block]VLAN 30: Segmen 32001:db8:abcd:2::/64 (1 Ho[Subnet Block]VLAN 40: Segmen 42001:db8:abcd:3::/64 (1 Ho[Subnet Block]
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Technical Accuracy Note: Calculation results are engineering estimates based on standard RFC/IEEE/ITU-T models. Actual field performance may vary due to protocol overhead, hardware specifications, and environmental factors.

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How to Use This Tool

  1. Enter an IPv6 address (for example 2001:db8:abcd:12::1) or a full CIDR notation (for example 2001:db8:abcd::/48) into the address input.
  2. Select the parent prefix length (such as /32 for ISP allocations, /48 for enterprise sites, /56 for branch offices, or /64 for a single LAN).
  3. Choose a target child prefix length (such as /64 or /56) to compute the number of child subnets and inspect the first 8 boundary-aligned blocks.

Worked Example: Enterprise Site /48 → /64 (2001:db8:abcd::/48)

Computed example: Site allocation 2001:db8:abcd::/48 (expanded: 2001:0db8:abcd:0000:0000:0000:0000:0000) spans 2001:db8:abcd:: through 2001:db8:abcd:ffff:ffff:ffff:ffff:ffff (2^80 = 1.208.925.819.614.629.174.706.176 addresses) and yields 65.536 standard /64 SLAAC subnets (Documentation (2001:db8::/32)).

Example Input
IPv6 Address or CIDR Prefix
2001:db8:abcd::1
Parent Prefix Length (0–128)
48 bits
Target Child Prefix (Subnet Split)
/64 — Standard LAN Subnet (SLAAC)
Calculated Output
Input IPv6 Address
2001:db8:abcd::1
CIDR Prefix
48 bits
Compressed IPv6 Address (RFC 5952)
2001:db8:abcd::1
Expanded IPv6 Address
2001:0db8:abcd:0000:0000:0000:0000:0001
IPv6 Network Prefix (CIDR)
2001:db8:abcd::/48
Compressed Network Address
2001:db8:abcd::
Expanded Network Address
2001:0db8:abcd:0000:0000:0000:0000:0000
First Address in Range
2001:db8:abcd::
Last Address in Range (Compressed)
2001:db8:abcd:ffff:ffff:ffff:ffff:ffff
Last Address in Range (Expanded)
2001:0db8:abcd:ffff:ffff:ffff:ffff:ffff
Prefix Mask (Compressed)
ffff:ffff:ffff::
Prefix Mask (Expanded)
ffff:ffff:ffff:0000:0000:0000:0000:0000
Host Bits
80 bits
Total IPv6 Addresses
1.208.925.819.614.629.174.706.176
Total Addresses Power
2^80
Number of /64 Subnets (SLAAC)
65.536
Subnet /64 Power
2^16
IP Address Type
Documentation (2001:db8::/32)
Reverse DNS Zone (ip6.arpa)
d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa
Target Child Prefix
64 bits
Child Subnet Count
65.536
Child Subnet Power
2^16

Core Concept & Engineering Notes

IPv6 addressing uses 128-bit identifiers written as eight 16-bit hexadecimal groups (hextets) separated by colons, as defined in RFC 4291. Because writing all 32 hexadecimal digits is error-prone during daily operations, RFC 5952 specifies a canonical compressed text representation: leading zeros within each hextet are removed, and the single longest consecutive run of all-zero hextets is collapsed into a double colon (::). Unlike IPv4, where engineers slice blocks down to tight host counts (/27, /28, /29) to conserve scarce addresses, IPv6 subnetting is designed around nibble boundaries (multiples of 4 bits) and a universal /64 LAN subnet size. Under RFC 4291 and RFC 4862, every end-user VLAN or wireless segment should be assigned a /64 prefix so Stateless Address Autoconfiguration (SLAAC) can operate reliably using a 64-bit Interface Identifier (IID). Regional Internet Registries (RIRs) and ISPs typically assign a /32 to service providers, a /48 to each enterprise site or data center, and a /56 to smaller branch or residential sites. A single /48 site prefix contains 2^(64 - 48) = 65,536 individual /64 subnets, while a /56 prefix provides 256 /64 subnets. This hierarchy allows network architects to allocate subnets cleanly along four-bit hex boundaries per building, floor, or security zone. For point-to-point router interconnects, RFC 6164 recommends configuring a /127 prefix (the IPv6 equivalent of RFC 3021 /31 in IPv4) to prevent Neighbor Discovery cache exhaustion and ping-pong routing loops, while reserving the underlying /64 block in the address plan. Beyond routable Global Unicast Addresses (GUA, 2000::/3), IPv6 defines Unique Local Addresses (ULA, fc00::/7 and fd00::/8 per RFC 4193) for private internal routing, Link-Local addresses (fe80::/10) required on every active interface for Neighbor Discovery Protocol (NDP) and routing protocols, Multicast (ff00::/8) which replaces IPv4 broadcast, and the reserved documentation prefix 2001:db8::/32 per RFC 3849.

How It Was Calculated (Formulas & Steps)

Frequently Asked Questions (FAQ)

Why should every end-user IPv6 LAN use a /64 prefix?

IETF standards (RFC 4291 and RFC 4862) split a 128-bit IPv6 address into a 64-bit network prefix and a 64-bit Interface Identifier (IID). Stateless Address Autoconfiguration (SLAAC) requires a 64-bit IID; prefixes longer than /64 break SLAAC on operating systems that do not support DHCPv6.

How many /64 subnets are inside a /48 or /56 prefix?

A /48 prefix provides 2^(64-48) = 2^16 = 65,536 individual /64 subnets. A /56 prefix provides 2^(64-56) = 2^8 = 256 individual /64 subnets.

What is the difference between GUA, ULA, and Link-Local IPv6 addresses?

Global Unicast Addresses (GUA, 2000::/3) are publicly routable on the global internet. Unique Local Addresses (ULA, fc00::/7, typically fd00::/8) are private addresses for internal networks similar to RFC 1918 in IPv4. Link-Local addresses (fe80::/10) are valid only on a single L2 link and are mandatory for Neighbor Discovery.

What prefix length should be used for router-to-router point-to-point links?

RFC 6164 recommends using a /127 prefix on point-to-point router links to prevent Neighbor Discovery exhaustion attacks, while reserving a full /64 out of your address plan so no other interface shares that /64 boundary.

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References & Notes

Last updated: Technically Verified · Industry Standard

Note: Results above are mathematical estimates based on standard formulas. Always verify against official vendor documentation and site conditions before production deployment.

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⚡ Ready-to-Use Scenario Presets:

/32 = Mask 255.255.255.255 • 1 Usable Hosts→
/32 Subnet Mask (255.255.255.255): Usable Hosts (1), Wildcard & IPv4 Range

Instant /32 CIDR reference: Subnet Mask is 255.255.255.255, Wildcard 0.0.0.0, Total IPs 1, and 1 Usable Hosts. Ideal for OSPF/BGP Router Loopback & Single Host Route.

/30 = Mask 255.255.255.252 • 2 Usable Hosts→
/30 Subnet Mask (255.255.255.252): Usable Hosts (2), Wildcard & IPv4 Range

Instant /30 CIDR reference: Subnet Mask is 255.255.255.252, Wildcard 0.0.0.3, Total IPs 4, and 2 Usable Hosts. Ideal for Point-to-Point Inter-Router WAN Link (2 Usable IPs).

/29 = Mask 255.255.255.248 • 6 Usable Hosts→
/29 Subnet Mask (255.255.255.248): Usable Hosts (6), Wildcard & IPv4 Range

Instant /29 CIDR reference: Subnet Mask is 255.255.255.248, Wildcard 0.0.0.7, Total IPs 8, and 6 Usable Hosts. Ideal for DMZ Public Server Sub-Block & BGP Peering (6 Hosts).

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🔬 Scientific Methodology & Normative Verification

Engineering Sensitivity Analysis, Response Curve & Technical Citation

✓ Calibrated to ISO/IEC/IEEE/TIA Clauses

1. 1. 3-Point Engineering Sensitivity & Tolerance Envelope

Comparative evaluation across theoretical laboratory bounds (0% loss), current nominal operating point, and conservative field safety bounds (±15% thermal/aging derating).

Evaluated Parameter Ideal Theoretical Bound Nominal Operating Point Conservative Safety Bound (±15%) Engineering Derating Rule
CIDR Prefix 43.2 48 55.2 Engineering Safety Margin (+15% Overhead)
Host Bits 72 80 92 Engineering Safety Margin (+15% Overhead)
Number of /64 Subnets (SLAAC) 58.98 65.54 75.37 Engineering Safety Margin (+15% Overhead)
Target Child Prefix 57.6 64 73.6 Engineering Safety Margin (+15% Overhead)
Child Subnet Count 58.98 65.54 75.37 Engineering Safety Margin (+15% Overhead)

2. 2. Mathematical Function Curve & Operating Coordinate (x₀, y₀)

MATHEMATICAL RESPONSE CURVE: CIDR PREFIX VS CIDR PREFIX Conservative Engineering Bound (+15% Safety Margin) 19.2 28.8 38.4 48 57.6 67.2 76.8 (x₀=48, y₀=48) Variabel Independen (x): CIDR Prefix

3. 3. Physical Constants & Normative Standard Clauses

R_loop(Cat6, 20°C) = 14 Ω/100m ANSI/TIA-568.2-D & IEEE 802.3bt

DC Loop Resistance for 23 AWG Solid Copper Cat6

Clause 6.4.1 / Table 145-1 — Nominal 23 AWG solid copper loop resistance at 20°C is 14.0 Ω/100m (TIA maximum ceiling is 18.8 Ω/100m).

α_opt(1490nm) = 0.25 dB/km ITU-T Rec. G.652.D & G.984.2

Single-Mode Optical Fiber Attenuation Coefficient at 1490 nm

Table 1 / Class B+ & C+ Optical Path — Typical G.652.D attenuation at 1490 nm downstream is 0.22–0.25 dB/km (1310 nm upstream: 0.35 dB/km).

α_Cu(20°C) = 0.00393 °C⁻¹ IEC 60028

Temperature Coefficient of Resistance for Copper

International Annealed Copper Standard — Used in thermal derating equation: R(T) = R_20 × [1 + 0.00393 × (T - 20)].

4. 4. Independent Technical & Academic Citation (IEEE / APA 7th / BibTeX / LaTeX)

Use the standard citation formats below to reference this computational methodology in engineering specifications, audit reports, or academic papers.

Format IEEE & APA 7th Edition

[IEEE] KalkuNet Engineering Team, "IPv6 Subnet Calculator and Prefix Planner (/48, /56, /64) — Standard Engineering Computational Instrument," KalkuNet Technical Suite, 2026. [Online]. Available: https://kalkunet.com/en/tools/ipv6-subnet-calculator. [Accessed: 2026-11-08].

[APA 7th] KalkuNet Engineering Team. (2026). IPv6 Subnet Calculator and Prefix Planner (/48, /56, /64) (Production release 2026-11-08) [Engineering computational software]. KalkuNet. https://kalkunet.com/en/tools/ipv6-subnet-calculator

BibTeX & LaTeX Derivation Snippet
@software{kalkunet_ipv6_subnet_calculator_2026,
  author = {{KalkuNet Engineering Team}},
  title = {{IPv6 Subnet Calculator and Prefix Planner (/48, /56, /64) --- KalkuNet Engineering Suite}},
  year = {2026},
  publisher = {KalkuNet.com},
  url = {https://kalkunet.com/en/tools/ipv6-subnet-calculator},
  urldate = {2026-11-08}
}

% LaTeX Engineering Derivation — Generated via https://kalkunet.com/en/tools/ipv6-subnet-calculator
\begin{equation}
  \text{Model: } Host Bits = 128 − Prefix
\end{equation}
% Parameter Input Terukur: address = 2001:db8:abcd:12::1, \; prefix = 48, \; childPrefix = 64
% Hasil Evaluasi Numerik : inputAddress = \mathbf{2001:db8:abcd:12::1}, \; prefix = \mathbf{48}, \; compressed = \mathbf{2001:db8:abcd:12::1}