CIDRGen — IPv4 & IPv6 Subnet Calculator & Netmask Generator

Subnet Calculator 100% CLIENT-SIDE

CIDR Notation (e.g. 192.168.1.0/24)

Subnet Breakdown & Ranges

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Guides

CIDR Cheat Sheet Subnetting Without Math CIDR Overlaps and Aggregation

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Frequently Asked Questions

Is my data safe on cidrgen.dev?

Yes. Subnet math is computed locally in your browser, so no address you enter is transmitted or stored.

How many usable hosts are in a /24 network?

254. A /24 has 32-24 = 8 host bits, so 256 addresses, minus the network address and the broadcast address. The calculator lists network, first usable, last usable, and broadcast addresses so you never have to do that subtraction by hand.

Does CIDRGen handle IPv6?

Yes. It accepts IPv6 prefixes alongside IPv4 CIDR notation and reports the network range and address count for each.

🧭 Pro Tips & Gotchas

Reading the output

Host counts are 2^(32-prefix); usable hosts subtract the network and broadcast addresses, so a /24 gives you 254 assignable addresses, not 256.
The private-space check flags RFC 1918 only: 10/8, 172.16/12, 192.168/16. A 100.64/10 (carrier-grade NAT) or 169.254/16 (link-local) block will read as "public" and that is correct, not a bug.
Wildcard mask is the bitwise inverse of the netmask — useful for ACLs that expect it (0.0.0.255 for /24).

Subnetting gotchas

/31 and /32 break the -2 rule: RFC 3021 makes point-to-point /31 links have 2 usable addresses, and /32 is a single host. Applying "total minus 2" universally reports 0 usable for both.
AWS, GCP, and Azure each reserve extra addresses beyond network/broadcast — typically the first and last plus a couple of hypervisor/VPN reservations — so a /24 in AWS yields 251 or 252 free, not 254. Plan capacity against the cloud's number, not the theoretical one.
A subnet's base address is always a multiple of its size. 10.0.1.128/25 is legal; 10.0.1.100/25 is not a network address even though every cloud console will accept the string and silently re-interpret it.
Prefix arithmetic is on the prefix, not the octet: 10.0.0.0/23 spans 10.0.0.0 – 10.0.1.255. Assuming boundaries land on .0/.255 is how overlap bugs get designed in.
IPv6 ignores all of the above: 64 is the default delegation, :1 is the router, there is no broadcast, and -2 never applies.
CIDR notation is prefix length, not dotted-mask input. 192.168.1.0/255.255.255.0 will fail; convert the mask to /24 first.