CIDR / Subnet Calculator

IPv4 and IPv6 subnets, ranges, splits and overlap checks in one place.

Runs locally · nothing uploaded
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Network
Address type
Binary

What it does

Type a network and everything is calculated as you type. The input can be a CIDR block (10.0.0.0/16, 2001:db8::/48), an address with a netmask (192.168.1.10 255.255.255.0, a wildcard mask such as 0.0.0.255 also works) or a range (10.0.0.5 - 10.0.0.77), which is turned into the minimal list of CIDR blocks.

  • Details: network, broadcast, first and last usable host, host count (including the 5 addresses Azure and AWS reserve per subnet), netmask, wildcard mask, reverse DNS zone, and a binary view with network and host bits coloured.
  • Address type: private (RFC 1918), CGNAT (100.64.0.0/10), loopback, link-local, multicast, documentation ranges, IPv6 ULA, link-local and global unicast, with the RFC for each.
  • Membership check: is 10.20.37.4 inside this network? Network and broadcast addresses are pointed out.
  • IPv6 done properly: 128-bit math, :: compression per RFC 5952, expanded form, zone IDs and IPv4-mapped addresses.

Split and plan

The Split tab divides a network into N equal subnets, into subnets of a given prefix, or by host counts (VLSM): list web 200, GatewaySubnet /27 and so on, and blocks are allocated largest first with the leftover space shown. Results export as CSV.

Overlap & free space checks a list of CIDRs (Docker networks, VPN routes, peered VNets, Kubernetes ranges) for conflicts, merges them into the smallest set of blocks and lists the free blocks left in a parent network.

Privacy

Everything is calculated in your browser. Your last inputs are kept in local storage only.