IP Subnet Calculator · IPv4/IPv6 CIDR + VLSM + Aggregator — Vectobox
Free IPv4/IPv6 subnet calculator with VLSM planner and CIDR aggregator. Correctly handles RFC 3021 /31, RFC 6164 /127, /32 host routes, and IPv6 (no broadcast). 100% browser, zero tracking.
Pure browser. Zero upload. Your addresses never leave this tab.
IPv4 CIDR done right
An IPv4 prefix is a /N slice of the 32-bit address space. We split into network, broadcast, first / last host, dotted-decimal mask and Cisco-style wildcard mask. /31 returns 2 usable addresses (RFC 3021 point-to-point links) instead of 0, and /32 is a single host route — not an error.
IPv6 without IPv4 baggage
IPv6 has no broadcast (RFC 4291 §2.1), so the result panel never shows a broadcast field. We canonicalize to RFC 5952 short form, expose the full 128-bit expanded form, and tag /127 as RFC 6164 inter-router links and /128 as a single address.
VLSM planner
Sort requirements by host count descending, then allocate the smallest sufficient prefix for each on a boundary-aligned cursor (First-Fit Decreasing). /31 is preferred over /30 for 2-host requirements (RFC 3021). A final pairwise overlap pass catches any planner bug.
CIDR aggregation (supernet)
Given a list of CIDRs, find the smallest covering prefix (RFC 4632 §5.1). We also report unused gap addresses inside that aggregate, so you can see exactly how loose the supernet is.
FAQ
- Why does /31 give 2 usable hosts?
- RFC 3021 (December 2000) defines 31-bit prefixes for IPv4 point-to-point links: both addresses are assignable, there is no broadcast or network reservation. Older tools that subtract 2 unconditionally are wrong for /31.
- Why is broadcast not shown for IPv6?
- RFC 4291 §2.1 explicitly states that IPv6 has no broadcast addresses — broadcast functionality is achieved with multicast. Displaying a broadcast field for IPv6 is a category error.
- What is VLSM and when should I use it?
- Variable Length Subnet Masking lets you carve a parent CIDR into right-sized child subnets so you do not waste addresses on small links. Use it whenever you allocate from a finite block (RFC 1918 space, leased prefix, lab network).
- What is the wildcard mask?
- Cisco IOS ACLs use the bitwise-NOT of the subnet mask: 255.255.255.0 → 0.0.0.255. We compute it across all 32 bits, so non-octet-aligned prefixes like /27 produce the correct 0.0.0.31.
- Are my IP addresses sent anywhere?
- No. All parsing and calculation runs locally in your browser tab. Nothing is logged, stored, or sent to a server.
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