d4 · tools · cidr

CIDR calculator.

IPv4 and IPv6 subnetting without the spreadsheet: ranges and masks, splitting, VLSM planning, aggregation and overlap checks. Everything runs in your browser; nothing you type leaves this page.

Subnet details

Accepts CIDR (10.20.0.0/22, 2001:db8::/48), address + netmask (10.20.0.0/255.255.252.0) or a bare address.

Address or CIDR block Calculate

Split a block

Divide a block into equal subnets at a longer prefix.

Block to split New prefix length Split

VLSM planner

Give a base block and the networks you need; each line is name, hosts. Largest networks are placed first, so nothing is wasted to alignment.

Base block

Subnets needed (one per line)

Plan

Aggregate

Paste a messy list of blocks and addresses; get back the minimal exact set of CIDR blocks. Useful for firewall rules and route summarisation.

Blocks or addresses (one per line)

Aggregate

Overlap check

Do two blocks collide? Handy before adding routes, VPN networks or firewall objects.

Block A Block B Check

IPv4 host counts follow convention: usable hosts exclude the network and broadcast addresses, /31 is treated as an RFC 3021 point-to-point pair and /32 as a host route. IPv6 counts are raw address counts. Classifications follow the IANA special-purpose address registries.