KBY Engineering Utilities

IPv4 and IPv6 CIDR Planner

Normalize IPv4/IPv6 networks, calculate exact ranges and audit batch allocations for containment, overlap, alignment and unused capacity.

Accuracy contract: Network, mask, range, containment and subnet results are integer bit operations over 32-bit IPv4 or 128-bit IPv6 values.

Inputs

Defined calculation contract

Primary CIDRCanonical IPv4 or IPv6 address with prefix length.Comparison CIDROptional same-family network for overlap and containment analysis.Split prefixOptional longer prefix. The planner reports the exact subnet count and previews the first 64 networks.Allocated CIDRsOptional allocation set, one CIDR per line. Audits alignment, containment, duplicates, overlaps and exact coverage within the primary network.

Exact output

Calculation result

10.42.16.0/20

Canonical network

255.255.240.0

Netmask

4096

Address count

10.42.16.0

First address

10.42.31.255

Last address

Host bits

Range semantics

Family: IPv4
Usable first: 10.42.16.1
Usable last: 10.42.31.254
Usable count: 4094

Comparison: 10.42.24.0/21

Overlaps: Yes
Primary contains comparison: Yes
Primary is contained by comparison: No

Split to /24: 16 networks

10.42.16.0/2410.42.17.0/2410.42.18.0/2410.42.19.0/2410.42.20.0/2410.42.21.0/2410.42.22.0/2410.42.23.0/2410.42.24.0/2410.42.25.0/2410.42.26.0/2410.42.27.0/2410.42.28.0/2410.42.29.0/2410.42.30.0/2410.42.31.0/24

Batch allocation audit

CIDRs supplied: 3
All contained: Yes
All network-aligned: Yes
Covered addresses: 2048
Unallocated addresses: 2048
Duplicate pairs: 0
Overlapping pairs: 0

Exact CIDR summary

10.42.16.0/21

IANA special-purpose matches · registry 2025-10-09

10.42.16.0/23 — Private-Use (10.0.0.0/8)
10.42.18.0/23 — Private-Use (10.0.0.0/8)
10.42.20.0/22 — Private-Use (10.0.0.0/8)

Versioned sources

IETF RFC 4632: CIDR IETF RFC 4291: IPv6 Addressing Architecture IETF RFC 3021: 31-Bit IPv4 Prefixes IANA IPv4 and IPv6 Special-Purpose Address Registries

Deterministic v2026.07.3 — Principal · 2–6 min