What this CIDR calculator does
A CIDR / subnet calculator turns a short notation like `192.168.1.0/24` into the full picture of a network: which addresses belong to it, where the usable host range starts and ends, what the dotted mask looks like, and how the bits split between the network and host parts.
This tool runs entirely in your browser. Nothing leaves the page. It supports IPv4 and IPv6 (auto-detected), works with both CIDR notation and the old "address plus dotted mask" form, and ships with three modes: calculate a single network, split a parent block into smaller subnets, and aggregate a messy list of CIDRs into the smallest set that covers them all.
A few things most online subnet calculators get wrong: they show /31 as having "0 usable hosts" (RFC 3021 says 2, for point-to-point links), they refuse to do supernet aggregation, and they hide the binary breakdown that actually helps you understand why a mask works the way it does. This one fixes all three.
How to use it
- Pick a mode at the top: Calculate for a single network, Split to break a parent into children, Aggregate to merge a list.
- Flip the IPv6 toggle if you are working with IPv6 addresses. IPv4 stays the default because most day-to-day questions are IPv4.
- Calculate mode: paste a CIDR like `10.0.0.0/24` or type an address plus a dotted mask (`10.0.0.0` + `255.255.255.0`). You get network, broadcast, first and last host, total addresses, wildcard mask, and a per-bit binary breakdown.
- Split mode: enter the parent CIDR and either a target prefix length (e.g. split a /22 into /24s) or a count of subnets (we round up to the next power of two). You get the full list with usable host counts.
- Aggregate mode: paste one CIDR per line. The tool sorts, merges overlapping or adjacent ranges, and returns the minimum CIDR set that covers everything. Useful for firewall rule cleanup.
- All math happens client-side. Big IPv6 ranges (like /32) are computed with BigInt so the address counts stay exact, not approximate.
When this is useful
Common situations where a CIDR calculator saves you a few minutes:
- Designing a new VPC or VLAN: you need to carve a /16 into smaller chunks, one per environment or team. Split mode shows you exactly how many subnets you get and how many hosts each has.
- Reading a firewall rule: an entry like `172.16.0.0/12` means "everything from 172.16.0.0 to 172.31.255.255". The calculator confirms the range so you do not allow more than you intended.
- Cleaning up an ACL: you have a long list of /24s and want to know whether they can be expressed as a few /20s or /16s. Aggregate mode does the merge for you.
- Understanding the dotted mask: a coworker writes `255.255.252.0` and asks "what prefix is this?". The calculator answers /22 and shows the bit pattern.
- Spotting an off-by-one: someone claims `10.0.0.0/24` includes `10.0.1.5`. It does not. The calculator shows the broadcast at `10.0.0.255`, full stop.
- Working with IPv6: address counts get huge fast (a /64 has 18 quintillion addresses). The tool uses BigInt so the numbers stay exact and you can compare them honestly.
- Studying for a networking cert: the binary view lets you see the network bits versus host bits, which is the part textbooks talk about but most online tools skip.
Related tools: DNS lookup, WHOIS lookup, What is my IP, IP info and ASN lookup.